Method of manufacturing a wind turbine blade having predesigned segment

ABSTRACT

A blade for a rotor of a wind turbine is manufactured with a root region with a substantially circular or elliptical profile closest to the hub, an airfoil region with a lift generating profile furthest away from the hub and a transition region having a profile gradually changing the root region to the airfoil region. A first blade design is used for the first base part on a first longitudinal section of an airfoil region of a second blade, so that an induction factor of the first base part on the second blade deviates from a target induction factor. The first longitudinal section of the second blade is provided with flow altering devices so as to adjust the aerodynamic properties of the first longitudinal segment to substantially meet the target induction factor at the design point on the second blade.

This is a National Phase Application filed under 35 U.S.C. 371 as anational stage of PCT/EP2010/056808, filed May 18, 2010, and claimingthe benefit from European Application No. 09160496.7, filed May 18,2009, the content of which is hereby incorporated by reference in itsentirety.

The present invention relates to a method of manufacturing a windturbine blade by using the design of a longitudinal segment of a firstwind turbine blade for the design of a second wind turbine blade,wherein the first wind turbine blade and the second wind turbine bladeeach comprise a longitudinally extending base part having: a profiledcontour comprising a pressure side and a suction side as well as aleading edge and a trailing edge with a chord extending between theleading edge and the trailing edge, the profiled contour generating alift when being impacted by an incident airflow, the profiled contour inthe radial direction being divided into a root region with asubstantially circular or elliptical profile closest to the hub, anairfoil region with a lift generating profile furthest away from thehub, and preferably a transition region between the root region and theairfoil region, the transition region having a profile graduallychanging in the radial direction from the circular or elliptical profileof the root region to the lift generating profile of the airfoil region.The invention further relates to a group of wind turbine bladescomprising at least a first wind turbine blade and a second wind turbineblade as well as a group of wind turbines comprising such groups ofblades.

Traditionally, modern wind turbine blades are designed by initiallydesigning the outer shape and the aerodynamic performance of the bladeitself in order to obtain the target loading and target axial inductionfor each radial section of the airfoil section of the blade. Firstafterwards, it is determined how to manufacture the blade in accordancewith the aerodynamic design specification for the blade. The aerodynamicshapes of such blades are typically complex with segments having doublecurvatured contours and several different airfoil shapes along theradial extent of the wind turbine blade. Accordingly, the manufacturingprocess of the blades as well as manufacturing mould parts for themanufacturing process become quite complex. Overall, the time forinitial start-up for developing the design of a new blade type to theproduct launch of the new blade type is long, and the overall productionand development costs are high. This process is more thoroughlydescribed in section 1 of the later description.

WO 01/14740 discloses ways of modifying wind turbine blade profiles inorder to prevent stall problems.

EP 2 031 242 discloses a blade element for mounting on a wind turbineblade in order to change the profile from an airfoil shape with apointed trailing edge to an airfoil profile with a truncated trailingedge.

DE 199 64 114 A1 discloses an airfoil profile, which is fitted with adivergent trailing edge in form of Gurney flap, which creates a periodicflow disturbance.

WO 02/08600 discloses a wind turbine blade provided with a rib as wellas vortex generators on the connection part or root part of the blade.

U.S. Pat. No. 5,088,665 discloses a wind turbine blade provided with aserrated trailing edge panel.

WO 2007/140771 discloses a wind turbine blade provided with turbulencegenerating strips in order to prevent stall and reduce noise emissions.

EP 1 944 505 discloses a wind turbine blade provided with vortexgenerators in airfoil portions having a relative thickness of 30%-80%.

DE 10 2006 017 897 discloses a wind turbine blade provided with aspoiler device in the root region and transition region of the blade.

WO 03/029644 discloses a method of designing a turbine blade for a underwater flow turbine by use of e.g. axial induction factor as a designparameter. The blade profiles are not provided with flow guidingdevices.

WO 03/098034 discloses a wind turbine provided with a hub extender. Theblade profiles are not provided with flow guiding devices.

US 2007/140858 discloses a modularly constructed blade including bondsections which are disposed away from the leading edge and trailing edgeof the assembled blade. The blade profiles are not provided with flowguiding devices.

US 2007/105431 discloses a modularly constructed blade including aplurality of stacked modular segments, where the segments are clampedtogether using cables. The blade profiles are not provided with flowguiding devices.

EP 0 100 131 discloses a method of manufacturing wind turbine bladesusing pultrusion or extrusion. The blade profiles are not provided withflow guiding devices.

It is an objective of the invention to obtain a new method ofmanufacturing a wind turbine blade, which overcomes or ameliorates atleast one of the disadvantages of the prior art or which provides auseful alternative.

According to a first object of the invention this is obtained by theafore-mentioned method comprising the steps of: a) taking a first bladedesign of a first base part of a first longitudinal section of theairfoil region of a first blade, b) using the first blade design for thefirst base part on a first longitudinal section of the airfoil region ofa second blade, so that an induction factor of the first base part onthe second blade without flow altering devices at a rotor design pointdeviates from a target induction factor, and c) providing the firstlongitudinal section of the second blade with first flow alteringdevices so as to adjust the aerodynamic properties of the firstlongitudinal segment to substantially meet the target induction factorat the design point on the second blade, wherein the first longitudinalsegment extends along at least 20% of a longitudinal extent of theairfoil region of the second blade.

By using a first base part with inherent non-ideal aerodynamiccharacteristics, which subsequently are compensated for by use of flowaltering devices, it becomes possible to achieve a much more simple basepart profile. Furthermore, the method makes it possible to achieve amodular blade design, in which the same base part can be used forseveral different blade types and blade lengths without impairing theaerodynamic properties of the airfoil section comprising said base part.Thus, it is possible to reuse the base part of an existing blade furtheroutboard on a larger/longer blade, or alternatively reuse the base partof an existing blade further inboard on a smaller/shorter blade. All inall, it is possible to make a blade design in such a way that the bladedesign of the airfoil region is put together from pre-designed sectionsand that blades of different lengths can be composed partly fromsections already existing from previous blades.

Further, a blade segment with for instance an inherent non-optimum twistor a non-optimum chordal length distribution has a number of advantageswith respect to manufacturing of the base part, since the shape of thebase part can be kept much simpler than the shapes of conventional,modern wind turbine blades with a length of more than 40 meters. Forinstance double curvatured blade profiles may be avoided. This alsomakes the production of mould parts for manufacture of the bladessimpler. All in all, the time for initial start-up for developing thedesign of a new blade type to the product launch of the new blade typemay be lowered significantly, and the overall production costs may alsobe lowered.

However, putting such a constraint on the design parameters of the bladesegment means that the blade segment deviates from the optimum designwith respect to aerodynamics, in which the twist and chord length has anon-linear dependency on the radial position of the blade section. Thus,such a blade segment will inherently be non-ideal with respect toaerodynamics and in particular in relation to the axial inductionfactor. This deviation is compensated for by using flow altering devicesin order to adjust the design lift and inflow properties to theappropriate near-optimum axial induction as a function of the bladeradius. However, the target axial induction factor may deviate from theaerodynamic optimum axial induction of ⅓ due to structure and loadingconsiderations.

The adjustment of the loading to another blade radius implies the needfor use of flow altering devices. Thus, the flow guiding devices areused to adjust the blade to the rotor design point so that it has anear-optimum inflow condition and lift coefficient.

All in all, it is seen that the inventive concept behind the idea is adeparture from the traditional process of designing modern wind turbineblades, where the outer shape and the aerodynamic performance of theblade is designed initially, and first afterwards it is determined howto plan the manufacturing of the blades in accordance with the designspecification. The invention provides a new design process, in which theproduction is optimised in relation to effective methods ofmanufacturing a base part of a wind turbine blade, and where the basepart of the blade is retrofitted with flow guiding devices in order toobtain the proper aerodynamic specifications. Thus, the base part of theblade may deviate substantially from the optimum aerodynamic design.

The target axial induction may be seen as an average over the entirelongitudinal extent of the first longitudinal segment, or it may be seenas an individual target for a plurality of smaller radial segmentswithin the first longitudinal segment. Yet again, it may be seen as anindividual target for each cross-section of the first longitudinalsegment of the blade.

Preferably, the first longitudinal segment also extends along at least20% of the longitudinal extent of the airfoil region of the first blade.

Thus, the blade comprises at least one longitudinal segment extendingalong a substantial part of the airfoil region of the blade. Accordingto a first embodiment, the airfoil region includes a blade tip region ofthe blade. According to a second embodiment, the blade further comprisesa blade tip region abutting the airfoil region. Thus, the blade tipregion may be seen as either part of the airfoil region or as a separatepart. Typically, the tip region covers the outer 5-10% of thelongitudinal extent of the airfoil region.

For wind turbines and wind turbine blades, the pressure side of theblade is also defined and known as the windward side or the upwind side,whereas the suction side is also defined and known as the leeward sideor the downwind side.

The rotor design point is defined as the point, where a powercoefficient of the wind turbine blade is maximum for a design wind speedand a design rotor speed. Thus, each section of the blade has a localdesign tip speed ratio defined as the design rotor speed multiplied bythe local blade section radius divided by the design wind speed. Thus,it is seen that the design point is the point where a wind turbine usingsuch wind turbine blades has its maximum efficiency at the wind speedfor which the wind turbine is designed. At the design point, the localblade section has a local chord, twist and airfoil shape, which at thelocal inflow results in a design lift coefficient. All parameters shouldbe chosen or adjusted with the flow altering devices in order to obtainthe target axial induction factor, which governs the power beingproduced by this blade section. The rotor design point is furtherexplained in section 1.3.

According to an advantageous embodiment, the first longitudinal segmentof the first blade is located in a first radial distance from a root endof the first blade, and wherein the first longitudinal segment of thesecond blade is located in a second radial distance from a root end ofthe second blade, and wherein the first radial distance is differentfrom the second radial distance. Thus, it is clear that the aerodynamicproperties cannot be identical for the first longitudinal segment of thetwo blades without the use of flow altering devices, since the twist andchord length of the different radial positions are changed.

According to another advantageous embodiment, the first base part isoptimised for operation at the first radial distance. That is, the firstbase part is optimised for the first blade, meaning that the first basepart in itself can achieve the target axial induction of the first bladeat the rotor design point.

According to yet another advantageous embodiment, the first base parthas a profile shape which is substantially a hybrid between a firstprofile shape for obtaining a target induction at a first radialdistance at the rotor design point and a second profile shape forobtaining a target induction at a second radial distance at the rotordesign point. In this case, both the first blade and the second bladehave to be retrofitted with flow altering devices according to step c)in order to obtain optimum aerodynamic performance at the rotor designpoint.

The first base part may have a profile shape for example beingsubstantially an average between the first profile shape and the secondprofile shape. Thus, the first base part has a design profile, which isapproximately an average between an optimum design for the first bladeand an optimum design for the second blade.

According to an advantageous embodiment, steps a) and b) are carried outby connecting a hub extender to the first blade. Thus, the entire firstblade is reused for the second blade by connecting an extra root sectionto the blade, thereby making the rotor diameter of a wind turbine usingthe blade larger.

According to a second aspect, the invention provides a group of windturbine blades comprising at least a first wind turbine blade and asecond wind turbine blade, wherein the first wind turbine blade and thesecond wind turbine blade each comprise a longitudinally extending basepart having: a profiled contour comprising a pressure side and a suctionside as well as a leading edge and a trailing edge with a chordextending between the leading edge and the trailing edge, the profiledcontour generating a lift when being impacted by an incident airflow,the profiled contour in the radial direction being divided into a rootregion with a substantially circular or elliptical profile closest tothe hub, an airfoil region with a lift generating profile furthest awayfrom the hub, and preferably a transition region between the root regionand the airfoil region, the transition region having a profile graduallychanging in the radial direction from the circular or elliptical profileof the root region to the lift generating profile of the airfoil region.According to the invention, the first wind turbine blade and the secondwind turbine blade comprise a first longitudinal segment having asubstantial identical first base part, wherein the first longitudinalsegment extends along at least 20% of a longitudinal extent of theairfoil region of the second wind turbine blade, and wherein aninduction factor of the first base part of the second blade without flowaltering devices at a rotor design point deviates from a target axialinduction factor, and wherein the first longitudinal segment of thesecond blade is provided with a number of first flow altering devicesarranged so as to adjust the aerodynamic properties of the firstlongitudinal segment to substantially meet the target axial inductionfactor at the rotor design point.

According to an advantageous embodiment the second wind turbine bladecomprises the base part of the first wind turbine blade and is furtherprovided with a hub extender.

According to an advantageous embodiment, the first base part has anaxial induction factor, which without flow altering devices deviates atleast 5% from a target axial induction factor at a design point, and thefirst longitudinal segment is provided with a number of first flowaltering devices arranged so as to adjust the aerodynamic properties ofthe first longitudinal segment to substantially meet the target axialinduction factor at the design point.

According to an advantageous embodiment, the first base part has aninherent non-ideal twist and/or chordal length, and wherein thecross-sectional profile is adapted to compensate for the non-ideal twistand/or chordal length by shifting the axial induction towards the targetaxial induction. This type of blade is particularly applicable fordesigning wind turbine blades having a base part with a simplifiedchordal distribution and/or twist. According to an advantageousembodiment, the first base part has an inherent non-ideal twist and/orchordal length, and wherein the cross-sectional profile is adapted tocompensate for the non-ideal twist and/or chordal length by shifting anaxial induction towards a target axial induction. However, putting sucha constraint on the design parameters of the blade segment means thatthe blade segment deviates from the optimum design with respect toaerodynamics. Thus, such a blade segment will inherently be non-idealwith respect to aerodynamics and in particular in relation to an optimumlift coefficient for the segment. This deviation is compensated for byusing flow altering devices in order to adjust the design lift to theappropriate near-optimum axial induction as a function of the bladeradius. The adjustment of the loading to another blade radius impliesthe need for use of flow altering devices.

According to another advantageous embodiment, the axial induction factorof the first longitudinal segment with flow altering devices deviates nomore than 2% from the target axial induction factor at the design point.Advantageously, the deviation is no more than 1% from the target axialinduction factor at the design point.

According to yet another advantageous embodiment, the induction factorof the first base part without flow altering means deviates from thetarget axial induction factor along substantially the entirelongitudinal extent of the first longitudinal segment.

According to one embodiment, the target axial induction factor issubstantially equal to the aerodynamic optimum target axial inductionfactor. Thereby, it is possible to substantially maximise the energyextracted from the wind and thus maximise the power production of a windturbine utilising such blades.

However, the target axial induction factor may lie in the intervalbetween 0.25 and 0.4, or between 0.28 and 0.38, or between 0.3 and 0.33.Thus, it is seen that the target axial induction factor—due tostructural and operational consideration—may deviate from thetheoretical optimum of ⅓.

According to another embodiment, the induction factor of the first basepart without flow altering devices at the design point deviates at least10%, or 20% or 30% from the target axial induction factor. In otherwords, the axial induction factor is shifted on average more than 10% byapplying the flow altering devices to the first longitudinal segment ofthe blade.

According to yet another embodiment, the first base part without flowaltering devices at the design point further deviates from a targetloading, and wherein the first flow altering devices are furtherarranged so as to adjust the aerodynamic properties of the firstlongitudinal segment to substantially meet the target loading at thedesign point. The target loading is in this regard considered to be theresultant air force or more accurately the resultant normal force to therotor plane influencing the particular blade section. The target loadingmay be seen as an average over the entire longitudinal extent of thefirst longitudinal segment, or it may be seen as an individual targetfor a plurality of smaller radial segments within the first longitudinalsegment. Yet again, it may be seen as an individual target for eachcross-section of the first longitudinal segment of the blade.

The loading of the first base part may without flow altering devices atthe design point deviate at least 5%, or 10%, or 20% or 30% from thetarget loading. In other words, the loading of the first longitudinalsegment is shifted on average over the entire longitudinal extent by atleast 5% or 10% by applying the flow altering devices to the firstlongitudinal segment of the blade.

Advantageously, the loading of the first longitudinal segment with flowaltering devices deviates no more than 2% from the target loading at thedesign point. Advantageously, the deviation is no more than 1% from thetarget loading at the design point.

According to an advantageous embodiment, the first base part has aninherent non-ideal twist, such as no twist, or a reduced twist comparedto a target blade twist. Such a base part is further simplified comparedto conventional blade shapes.

According to another advantageous embodiment, the first longitudinalsegment in the radial direction is divided into: a plurality of radialsections, each radial section having an individual average operatingangle of attack for the design point and having a sectional airfoilshape, which without the first flow altering devices has a sectionaloptimum angle of attack, wherein the first flow altering devices areadapted to shift the optimum angle of attack of the sectional airfoilshape towards the average operating angle of attack for the radialsection.

According to yet another advantageous embodiment, the first base parthas a twist, which is non-ideal along substantially the entirelongitudinal extent of the first longitudinal segment. Accordingly, theinherent twist differs from the ideal twist along substantially theentire longitudinal extent of the segment, but the inherent twist may atvarious radial positions be identical to the optimum twist. Thus, graphsrepresenting the ideal twist and the inherent twist may at certain pointcross each other.

The invention is particularly suited for optimising the performance ofblades having substantially no twist, i.e. blades which have notinherently been designed to compensate for the local inflow velocity dueto the local varying velocity of the blade. Accordingly, the flowaltering devices can be utilised to vary the shift angle in thelongitudinal direction of the blade, so that the shift angle correspondsto a virtual twist of the blade in order to compensate for the localinflow velocity due to the local varying velocity of the blade. However,the invention can also be utilised with other types of blades andparticularly on blades having a reduced overall twist angle compared tothe optimum. Therefore, the blade according to one embodiment of theinvention has an airfoil region with a twist of less than 8 degrees. Inother words, the orientation of the chord plane changes less than 8degrees in the radial direction of the blade. However, the blade maystill be pre-bent and/or tapered in the radial direction of the blade.According to an alternative embodiment the twist is less than 5 degrees,or 3 degrees, or even less than 2 degrees. Thereby, it is possible toprovide a wind turbine blade with a much less complex profile than aconventional wind turbine blade, which typically has an airfoil sectionwith a maximum twist between 10 and 12 degrees, sometimes even 15degrees, and providing the blade with flow altering devices in order tocompensate for the “missing” twist or providing the “remaining” twist.

However, according to a particularly advantageous embodiment, theairfoil region of the blade is substantially straight. In other words,the orientation of the chord plane is substantially the same in theentire radial direction of the blade. Accordingly, each radial sectioncan be provided with flow altering devices in order to optimise the liftof the substantially straight blade. This provides a large number ofpossibilities for the design of blades, since the blades can be designedwithout twist and still be optimised for the local radial velocity ofthe blade during normal use, i.e. at the design point. This means thatthe blade can be manufactured from individual sectional blade parts,e.g. as individual blade parts, which are mutually connected afterwards,or by use of sectional mould parts as for instance shown in DE 198 33869. Alternatively, a given blade can be fitted with a hub extenderwithout changing the direction of the chord for a given radial positionof the blade. This also makes it possible to design the blade without anideal double curvatured pressure side, i.e. without the need of havingboth a convex and a concave surface profile on the pressure side of theblade. In this situation, the flow altering devices may be utilised tocompensate for a non-ideal profile. Thus, the mould assemblies can bemanufactured with a much simpler shape. Also, such a blade may make itpossible to manufacture the blade via simpler fabrication methods, suchas extrusion or the like.

The first derivative of the twist is reduced with increasing distancefrom the hub. Therefore, the twist of the outer part of the blade, i.e.near the tip, is smaller than the twist of the inner part of the blade.Consequently, not all blades need to be provided with flow alteringdevices near the tip end. However, preferably at least the inner 40%,50%, 60%, 70%, or 75% of the airfoil area is provided with radial bladesection having flow altering devices. The inflow in the tip region maybe compensated for by altering the blade pitch angle and/or therotational speed of the rotor.

According to a particularly advantageous embodiment, the first base parthas a substantially constant twist, e.g. substantially no twist, meaningthat the chord of the first base part is substantially arranged in thesame direction. Thus, the first base part may be substantially straight.

According to another advantageous embodiment, the first base part has atwist being linearly dependent on a radial position. That is, the twistangle or the chord angle varies linearly in the spanwise or longitudinaldirection of the first longitudinal segment. Such a blade segment may befitted to follow the ideal twist as closely as possible, but has anumber of advantages with respect to obtaining a feasible modulardesign, where the first base part is reused on another blade type orwhere it is “connected” to a second base part of a second longitudinalsegment and having another dependency on the radial position, optionallyvia an intermediate, transitional blade segment. In other words, such ablade segment has a number of advantages with respect to obtaining amodular design of the blade.

According to a first embodiment, the first base part has an inherenttwist angle so that the first base part without flow altering devices atthe rotor design point has an inflow angle, which is lower than theoptimum inflow angle along the entire longitudinal extent of the firstlongitudinal segment. In this situation, a single type of flow alteringdevices may be sufficient to accommodate for the non-ideal aerodynamicstructure of the first base part.

According to a second embodiment, the first longitudinal segment has aninherent twist angle so that the first base part without flow alteringdevices at the rotor design point comprises a first segment, in whichthe inflow angle is lower than the optimum inflow angle, and a secondsegment, in which the inflow angle is higher than the optimum inflowangle. In this situation, it may be necessary to employ different typesof flow altering devices in order to accommodate for the non-idealaerodynamic structure of the first base part. Such a blade may occur, ifthe inherent twist of the first base part is linearly dependent on theradial distance from the hub and where the inherent twist “crosses” theideal twist, which has a non-linear dependency on the radial position.Since the ideal twist has an inverse proportional dependency on theradial distance from the hub, a blade having a first base part with aninherent linear twist dependency may comprise—seen from the hub towardsthe blade tip—a first segment having an inherent twist being lower thanthe ideal twist, a juxtaposed second segment having an inherent twistbeing higher than the ideal twist, and a juxtaposed third segment havingan inherent twist being lower than the ideal twist.

Advantageously, the root mean square difference over the longitudinalextent of the first longitudinal section between the average inflowangle and the optimum inflow of attack at the design point is more than1 degree, or more than 2 degrees, or more than 2.5 degrees for the firstlongitudinal segment without flow altering devices. Thus, the root meansquare difference is calculated as an absolute spatial deviation in thelongitudinal direction of the blade. This deviation is further observedover a given time interval, e.g. one full cycle for a wind turbinerotor. Advantageously, the root mean square difference over thelongitudinal extent of the first longitudinal section between theaverage inflow angle and the optimum inflow angle at the design point isless than 1 degree, or less than 0.5 degrees for the first longitudinalsegment with the flow altering means.

According to an advantageous embodiment, the first base part has aninner dimension that varies linearly in the radial direction of theblade in such a way that an induction factor of the first base partwithout flow altering devices at a rotor design point deviates from atarget induction factor. Such a base part simplifies the design evenfurther compared to the design of conventional blade designs.

In the following a number of advantageous embodiments having linearlyvarying inner dimensions are described and which are simplified comparedto conventional, modern wind turbine blades.

According to a first advantageous embodiment, the length of the chord ofthe first base part varies linearly in the radial direction of theblade.

According to another advantageous embodiment, the first base part has athickness, which varies linearly in the radial direction of the blade.The thickness of the blade is in this regard defined as being themaximum thickness of the blade, i.e. for each cross-sectional profilebeing the maximum distance between the suction side and the pressureside of the blade (in a direction perpendicular to the cross-sectionairfoil chord).

According to yet another advantageous embodiment, the first base parthas a constant relative thickness. That is, the ratio between thethickness and the chord is constant along the entire longitudinal extentof the first longitudinally extending section of the blade. In principlethe relative profile may be varying in the longitudinal direction of theblade; however, according to an advantageous embodiment the first basepart comprises a constant relative profile.

In one embodiment, the first base part comprises a constant relativeprofile along the entire extent of the first longitudinally extendingsection. That is, every cross-section of the first base part has thesame relative airfoil profile or overall shape.

In another embodiment, the first base part has a constant chord length.This means that the chord length is constant along the entire extent ofthe first longitudinally extending section, or in other words that theleading edge and trailing edge of the first base part are parallel. Sucha constraint entails a significant deviation from a target axialinduction factor at the design point, but may significantly simplify theproduction of the blade as well as the design and fabrication of mouldsfor manufacturing the blade.

In yet another embodiment, the first base part has a constant thickness.

In a particular advantageous embodiment, the first base part comprises aplurality of longitudinal segment, each having a separate linearlyvarying dependency in the radial direction of the blade. Thus, it is forinstance possible to design a blade, which has a piece-wise linearlyvarying chord length. Each longitudinal segment should extend along atleast 20% of the longitudinal length of the airfoil region.

According to an advantageous embodiment, the first base part is providedwith a linear pre-bend. Thereby, the angular orientation of the basepart in relation to the pitch axis may be linearly dependent on thelocal blade radius. Alternatively, the transverse deviation from thepitch axis may be linearly dependent on the local blade radius. Thereby,it is possible to fit the pre-bend of individual blade segments in orderto obtain a pre-bent blade.

According to another advantageous embodiment, the first base part ispre-bent, and the airfoil region comprises longitudinal segmentscomprising base parts with no pre-bending. Thus, the pre-bend may belocated in one or two segments of the blade only, for instance theoutboard part of the airfoil region and/or in the root region.

According to an advantageous embodiment, the first base part is apultruded or extruded profile. Such base parts are feasible tomanufacture due to the linearly varying inner dimensions and simplifythe manufacturing process significantly.

According to an advantageous embodiment, the first base part has across-sectional profile, which when being impacted by an incidentairflow at an angle of attack of 0 degrees has a lift coefficient, whichis 0 or less. A positive lift is defined as a lift coefficient having alift component directed from the pressure side (or upwind/windward side)towards the suction side (or downwind/leeward side) of the blade. Anegative lift is defined as a lift coefficient having a lift componentdirected from the suction side (or downwind/leeward side) towards thepressure side (or upwind/windward side) of the blade.

Thus, the base part has a cross-sectional profile having an aerodynamicrelationship between the lift coefficient and the angle attack, whichwhen being plotted in a coordinate system with the lift coefficient as afunction of the angle of attack crosses the origin of the coordinatesystem or crosses the lift coefficient axis at a negative value. Inother words, the lift coefficient changes sign at a positive angle ofattack or at an angle of zero degrees, i.e. at a non-negative angle ofattack.

Such a base part will in itself have inherent non-optimum aerodynamicproperties for a conventional wind turbine blade having a profile, whichis twisted in the radial direction of the blade. However, the use ofprofile with such properties makes it possible to simplify otherproperties of the blade, such as the twist or the chordal shape of theblade. For example, it is made possible to provide a longitudinalsegment having no or a linear twist and/or having a linearly varyingchord length in the radial direction of the blade. However, putting suchconstraints on the design of the base part of the blade will inherentlyentail that the segment deviates substantially from the near-optimumtarget axial induction of that segment. In order to compensate for suchdeviations, it is necessary to change the overall inflow properties andthe lift coefficient of the segment. However, since the novel profilehas a relationship between lift coefficient and angle of attack, whichdiffers significantly from conventional blade profiles, this may besufficient to even out the deviations or at least change the axialinduction towards the target axial induction so that the flow alteringdevices only have to change the axial induction slightly.

Thus, the blade comprises at least one longitudinal segment extendingalong a substantial part of the airfoil region of the blade. Accordingto a first embodiment, the airfoil region includes a blade tip region ofthe blade. According to a second embodiment, the blade further comprisesa blade tip region abutting the airfoil region. Thus, the blade tipregion may be seen as either part of the airfoil region or as a separatepart. Typically, the tip region covers the outer 5-10% of thelongitudinal extent of the airfoil region.

In an example, where the first longitudinal segment has a zero twist ora twist being lower than the near-optimum twist, the novel profile (withthe above-mentioned relationship between lift coefficient and angle ofattack) compensates for the “lack” of twist, since the angle of attackhas to be higher than a conventional profile in order to obtain theright target characteristics, e.g. with respect to the necessary liftcoefficient in order to obtain the correct axial induction.

The use of the novel profile makes it feasible to achieve a modularblade design, in which the base part can be used for several differentblade types and blade lengths. Thus, it is possible to reuse the basepart of an existing blade further outboard on a larger/longer blade, oralternatively reuse the base part of an existing blade further inboardon a smaller/shorter blade. All in all, it is possible to make a bladedesign in such a way that the blade design of the airfoil region is puttogether from pre-designed sections and that blades of different lengthscan be composed partly from sections already existing from previousblades.

Overall, the shape of the base part can be kept much simpler than theshape of conventional, modern wind turbine blades with a length of morethan 40 meters. For instance double curvatured blade profiles may beavoided. This also makes the production of mould parts for manufactureof the blades simpler. All in all, the time for initial start-up fordeveloping the design of a new blade type to the product launch of thenew blade type may be lowered significantly, and the overall productioncosts may also be lowered.

Thus, according to an advantageous embodiment, the first base part hasan inherent non-ideal twist and/or chordal length, and wherein thecross-sectional profile is adapted to compensate for the non-ideal twistand/or chordal length by shifting an axial induction towards a targetaxial induction. However, putting such a constraint on the designparameters of the blade segment means that the blade segment deviatesfrom the optimum design with respect to aerodynamics. Thus, such a bladesegment will inherently be non-ideal with respect to aerodynamics and inparticular in relation to an optimum lift coefficient for the segment.This deviation is compensated for by using flow altering devices inorder to adjust the design lift to the appropriate near-optimum axialinduction as a function of the blade radius. The adjustment of theloading to another blade radius implies the need for use of flowaltering devices.

Thus, according to another advantageous embodiment, the firstlongitudinal segment is provided with a number of first flow alteringdevices arranged so as to adjust the aerodynamic properties of the firstlongitudinal segment to substantially meet a target axial inductionfactor at a rotor design point.

In the following, a number of advantageous embodiments are described,all of which provide the desired relationship between lift coefficientand angle of attack.

According to an advantageous embodiment, the first base part has across-sectional profile having a camber line and a chord line with achord length, and wherein the average difference between the chord lineand a camber line of the cross-sectional profile is negative over theentire chord length. That is, the camber is on average, when seen overthe entire length of the chord, closer to the pressure side of the bladethan to the suction side of the blade.

According to another embodiment, the camber line is closer to thepressure side than the suction side over the entire length of the chord.The camber and the chord are of course coinciding at the leading edgeand at the trailing edge.

According to an alternative embodiment, the first base part has across-sectional profile having a camber line and a chord line with achord length, wherein the camber line and the chord line are coincidingover the entire length of the chord. That is, the cross-sectionalprofile is symmetric about the chord. Such a profile is highlyadvantageous from a manufacturing point of view.

All in all, a first base part comprising: a linear chord, a linearthickness, and a twist, which varies linearly or is constant in theradial direction of the blade, has a number of advantages when designinga modular assembled blade and in respect to manufacturing such blades.

According to a third aspect, the invention provides a group of windturbines comprising at least a first wind turbine and a second windturbine, wherein the first wind turbine comprises a rotor with a number,preferably two or three, of first wind turbine blades according to thesecond aspect of the invention, and the second wind turbine comprises arotor with a number, preferably two or three, of first wind turbineblades according to the second aspect of the invention.

Preferably, the length of the wind turbine blade is at least 40 meters,or at least 50 meters, or at least 60 meters. The blades may even be atleast 70 meters, or at least 80 meters. Blades having a length of atleast 90 meters or at least 100 meters are also possible.

According to an advantageous embodiment, the blade and in particular thefirst base part comprise a shell structure made of a composite material.The composite material may be a resin matrix reinforced with fibres. Inmost cases the polymer applied is thermosetting resin, such aspolyester, vinylester or epoxy. The resin may also be a thermoplastic,such as nylon, PVC, ABS, polypropylene or polyethylene. Yet again theresin may be another thermosetting thermoplastic, such as cyclic PBT orPET. The fibre reinforcement is most often based on glass fibres orcarbon fibres, but may also be plastic fibres, plant fibres or metalfibres. The composite material often comprises a sandwich structureincluding a core material, such as foamed polymer or balsawood.

According to another advantageous embodiment, the blade comprises alongitudinal extending reinforcement section comprising a plurality offibre layers. The reinforcement section, also called a main laminate,will typically extend through the first base part of the firstlongitudinal segment.

According to an advantageous embodiment, the first longitudinal segmentextends along at least 25%, or 30%, or 40%, or 50%, of the airfoilregion. The first longitudinal segment may even extend along at least60%, 70% or 75% of the airfoil region. The extent of the firstlongitudinal segment may even be up to 100%, when the tip region isconsidered not being part of the airfoil region. However, the firstlongitudinal segment may as such be restricted to being part of theairfoil region, in which a near-optimum theoretical aerodynamicperformance at the design point may be achieved. This excludes the tippart, the root section, and the transitional section, which due to loadand structural considerations always will differ significantly from thenear-optimum theoretical aerodynamic performance.

Advantageously, the airfoil region may further comprise a longitudinallyextending transitional segment. The transitional segment—not to beconfused with the transition region of the blade—may extend radiallyalong 5-10% of the airfoil region, and is utilised in the airfoil regionto obtain a gradual transition between two longitudinally extendingsegments according to the invention. Thus, it is recognised that theblade may comprise a number of longitudinally extending sectionsextending along a substantial part of the blade and a number oftransitional segments. As an example, the outer part of the blade maycomprise a first longitudinally extending blade segment extending alongapproximately 40% of the airfoil region, a transitional segmentextending along approximately 10% of the airfoil region, a secondlongitudinally extending blade segment extending along approximately 40%of the airfoil region, and finally a blade tip section extending alongapproximately 10% of the airfoil region.

According to an advantageous embodiment, the first longitudinal segmentis provided at an inboard position of the airfoil region, i.e. in a partnearest the transition region or root region, preferably within twometers of the transition region of the root region, and more preferablyadjoining the optional transition region or the root region. The blademay be provided with additional longitudinal segments juxtaposed to thefirst longitudinal segment. All of these should extend along at least25% of the longitudinal extent of the airfoil region.

Advantageously, the flow guiding means comprises a multi elementsection, such as a slat, or a flap, i.e. the flow guiding meanspreferably comprises multi-element parts for changing the profilecharacteristics of different blade segments. The multi element sectionis adapted to alter the inflow properties and the loading of the firstlongitudinal segment of the blade. Preferably, the multi element sectionalters at least a substantial part of the first longitudinal segment,e.g. along at least 50% of the first longitudinal segment. Thereby, itis possible to change a number of design parameters, such as the designlift, the camber and the angle of attack for the segment, from a basedesign (of the first base part), which has an inherently non-optimumdesign from an aerodynamic point of view with respect to suchparameters, but which is optimised from a manufacturing point of view.Thus, it is possible to retrofit the multi-element parts to the firstbase part in order to optimise the aerodynamics. Accordingly, one ormore of the number of first flow altering devices may be arranged in theproximity of and/or along the leading edge of the first base part.Further, one or more of the number of flow altering devices may bearranged in the proximity of and/or along the trailing edge of the firstbase part. Thus, the overall profile may become a multi-element profilehaving at least two separate elements. Accordingly, the first base partmay be constructed as a load carrying part of the blade, whereas theflow guiding means are used to optimise the aerodynamics with respect tomatching the local section aerodynamic characteristics to the rotordesign point.

The multi element section may be arranged in a fixed position inrelation to the first base part. Thereby, the blade has permanently orsemi-permanently been adjusted in order to compensate for the non-idealprofile of the first base part. Alternatively, the multi element sectionmay be actively adjusted in relation to the first base part. Thus, thedesign parameters may be adjusted actively, e.g. according to theoperational conditions for the wind turbine. The first flow guidingmeans or the multi element section may be translational and/orrotational operational or adjustable in relation to the first base part.

According to one advantageous embodiment, the number of first flowaltering devices comprises a multi element section having an airfoilprofile with a chord extending between a leading edge and a trailingedge. This multi element section may be formed as an airfoil having achord length in the interval of 5% to 30% of a local chord length of thefirst base part. Alternatively, the afore-mentioned profile element hasa maximum inner cross-sectional dimension, which corresponds to 5% to30% of the chord length of the first base part.

According to a first embodiment, the number of first flow guiding meansor the structural profile element is arranged with a distance to thefirst base part. Alternatively, the structural profile element may beconnected to the surface of the first base part, thus as such alteringthe surface envelope of the base part itself.

According to yet another embodiment, the first base part has a surfacearea that is at least 5, or 7 times greater than the total surface ofthe number of flow altering devices.

Yet again, the flow guiding device may be adjustable in order topassively eliminate variations from inflow variations.

The flow altering devices may also comprise a surface mounted element,which alters an overall envelope of the first longitudinal segment ofthe blade. Advantageously, the surface mounted element is arranged inproximity of the leading edge and/or the trailing edge of the first basepart.

The flow altering devices may also comprise boundary layer controlmeans, such as holes or a slot for ventilation, vortex generators and aGurney flap. Preferably, the boundary layer control means are used incombination with the multi element sections or the surface mountedelements. Multi element sections or surface mounted elements aretypically necessary for achieving the large shift in the axial inductionfactor, i.e. for rough adjustment to the target. However, the boundarylayer control means may be utilised in order to fine adjust the axialinduction factor to the target.

Advantageously, the blade comprises a number of modular blade sections.The first longitudinal segment may for instance be such a blade section.The blade may also be a dividable or split blade, in which case theblade may be divided at one end of the first longitudinal segment.According to a first advantageous embodiment, the modular blade sectionscomprise a root section, the first longitudinal segment and a tipsection. According to a second advantageous embodiment, the root sectioncomprises the root region and the transition region. According to athird advantageous embodiment, the blade further comprises an extendersection for extending the length of the blade, preferably added to theroot section of the blade, such as a hub extender.

According to a further aspect, the invention provides a systemcomprising a group of root sections, optionally a group of extendersections, a group of airfoil sections including the first base part, anda group of tip sections. According to an advantageous embodiment, onemodular blade section from the group of root sections, optionally atleast one modular blade section from the group of extender sections, atleast one modular blade section from the group of airfoil sections andone modular blade section from the group of tip sections can be combinedand assembled, so as to form blades with different lengths.

According to yet another aspect, the invention provides a wind turbinecomprising a rotor including a number of blades, preferably two orthree, according to any of the afore-mentioned embodiments.

Advantageously, the wind turbine comprises a substantially horizontalaxis rotor shaft. Preferably, the wind turbine is operated in an upwindconfiguration, e.g. according to the “Danish concept”.

The invention is explained in detail below with reference to anembodiment shown in the drawings, in which

FIG. 1 shows a wind turbine,

FIG. 2 shows a schematic view of a wind turbine blade according to theinvention,

FIG. 3 shows a schematic view of an airfoil profile,

FIG. 4 shows a schematic view of flow velocities and aerodynamic forcesat an airfoil profile,

FIG. 5 shows a schematic view of a blade consisting of different bladesections,

FIG. 6 a shows a power curve versus wind speed for a wind turbine

FIG. 6 b shows a rotor speed curve versus wind speed for a wind turbine

FIG. 6 c shows a blade tip pitch curve versus wind speed for a windturbine

FIG. 7 shows a velocity vector triangle for a section on a wind turbineblade,

FIGS. 8 a and 8 b show graphs of inflow and blade loading, respectively,as a function of a local blade radius,

FIG. 9 shows a first embodiment of a blade according to the invention,

FIG. 10 shows a second embodiment of a blade according to the invention,

FIG. 11 shows a third embodiment of a blade according to the invention,

FIGS. 12 a-c and FIGS. 13 a-c illustrate compensatory measures forcorrecting non-optimum twist,

FIGS. 14 a-c and FIGS. 15 a-c illustrate compensatory measures forcorrecting non-optimum chordal length,

FIG. 16 shows the operating point for an actual blade section of a windturbine blade compared with the airfoil section design point.

FIGS. 17 a-17 e show the cross-section of a blade provided withventilation holes and the effect of using ventilation,

FIGS. 18 a-18 c show the cross-section of a blade provided with surfacemounted elements and the effect of using surface mounted elements,

FIG. 19 a shows the cross-sections of blades provided with multi elementprofiles and the effect of using such profiles,

FIGS. 19 b-d show different means of locating multi element profiles inrelation to a blade cross section,

FIGS. 20 a and 20 b show the cross-sections of a blade provided with aGurney flap and the effect of using a Gurney flap,

FIGS. 21 a-21 c show the cross-section of a blade provided with vortexgenerators and the effect of using vortex generators,

FIGS. 22 a and 22 b show the cross-sections of a blade provided with aspoiler element and the effect of using a spoiler element,

FIG. 23 a shows graphs of the average and optimum angle of attack as afunction of the radial distance from a hub,

FIG. 23 b shows a graph of the shift angle as a function of the radialdistance from a hub,

FIG. 23 c shows graphs of the relationship between the drag coefficientand the lift coefficient and the relationship between the angle ofattack and the lift coefficient for an outer part of a blade accordingto the invention, and

FIG. 23 d shows graphs of the relationship between the drag coefficientand the lift coefficient and the relationship between the angle ofattack and the lift coefficient for an inner part of a blade accordingto the invention.

FIGS. 24 a-g show graphs illustrating different embodiments of bladeshaving linearly dependent twist and/or chord,

FIG. 25 shows a fourth embodiment of a blade according to the invention,

FIG. 26 shows a graph of an embodiment of a blade having a linearprebend,

FIG. 27 shows a blade profile having a double curvatured pressure side,

FIG. 28 shows a blade profile without a double curvature,

FIG. 29 shows a graph of an embodiment of a blade having zero camber,

FIG. 30 shows a symmetric blade profile,

FIG. 31 shows a graph of an embodiment of a blade having a negativecamber,

FIG. 32 shows a first blade profile with a negative camber,

FIG. 33 shows a second blade profile with a negative camber,

FIG. 34 illustrates the principle of using a common blade section fortwo different types of wind turbine blades,

FIG. 35 shows the principle of using a hub extender,

FIG. 36 illustrates the principle of adjusting blade characteristics toa target value,

FIG. 37 shows an example of a chord length distribution,

FIG. 38 shows a comparison between the twist of a transformable bladeand that of an existing blade,

FIG. 39 shows graphs of the inflow angle for different blades and windspeeds,

FIG. 40 shows graphs of the lift coefficient for different blades andwind speeds,

FIG. 41 shows graphs of the axial induction factor for different bladesand wind speeds,

FIG. 42 shows graphs of the relative thickness distribution fordifferent blades,

FIG. 43 shows transformable blades having a shared outboard base part,

FIG. 44 shows an example of chord length distributions for transformableblades,

FIG. 45 shows graphs of the inflow angle for transformable blades,

FIG. 46 shows graphs of the lift coefficient for transformable blades,

FIG. 47 shows graphs of the inflow angle for other transformable blades,

FIG. 48 shows graphs of the lift coefficient for other transformableblades,

FIG. 49 shows an example of staggered transformable blades,

FIG. 50 shows another example of chord length distributions fortransformable blades,

FIG. 51 shows graphs of the inflow angle for transformable blades, and

FIG. 52 shows graphs of the lift coefficient for transformable blades,

FIG. 1 illustrates a conventional modern upwind wind turbine accordingto the so-called “Danish concept” with a tower 4, a nacelle 6 and arotor with a substantially horizontal rotor shaft. The rotor includes ahub 8 and three blades 10 extending radially from the hub 8, each havinga blade root 16 nearest the hub and a blade tip 14 furthest from the hub8. The rotor has a radius denoted R.

FIG. 2 shows a schematic view of a first embodiment of a wind turbineblade 10 according to the invention. The wind turbine blade 10 has theshape of a conventional wind turbine blade and comprises a root region30 closest to the hub, a profiled or an airfoil region 34 furthest awayfrom the hub and a transition region 32 between the root region 30 andthe airfoil region 34. The blade 10 comprises a leading edge 18 facingthe direction of rotation of the blade 10, when the blade is mounted onthe hub, and a trailing edge 20 facing the opposite direction of theleading edge 18.

The airfoil region 34 (also called the profiled region) has an ideal oralmost ideal blade shape with respect to generating lift, whereas theroot region 30 due to structural considerations has a substantiallycircular or elliptical cross-section, which for instance makes it easierand safer to mount the blade 10 to the hub. The diameter (or the chord)of the root region 30 is typically constant along the entire root area30. The transition region 32 has a transitional profile 42 graduallychanging from the circular or elliptical shape 40 of the root region 30to the airfoil profile 50 of the airfoil region 34. The chord length ofthe transition region 32 typically increases substantially linearly withincreasing distance r from the hub.

The airfoil region 34 has an airfoil profile 50 with a chord extendingbetween the leading edge 18 and the trailing edge 20 of the blade 10.The width of the chord decreases with increasing distance r from thehub.

It should be noted that the chords of different sections of the bladenormally do not lie in a common plane, since the blade may be twistedand/or curved (i.e. pre-bent), thus providing the chord plane with acorrespondingly twisted and/or curved course, this being most often thecase in order to compensate for the local velocity of the blade beingdependent on the radius from the hub.

FIG. 3 shows a schematic view of an airfoil profile 50 of a typicalblade of a wind turbine depicted with the various parameters, which aretypically used to define the geometrical shape of an airfoil. Theairfoil profile 50 has a pressure side 52 and a suction side 54, whichduring use—i.e. during rotation of the rotor—normally face towards thewindward (or upwind) side and the leeward (or downwind) side,respectively. The airfoil 50 has a chord 60 with a chord length cextending between a leading edge 56 and a trailing edge 58 of the blade.The airfoil 50 has a thickness t, which is defined as the distancebetween the pressure side 52 and the suction side 54. The thickness t ofthe airfoil varies along the chord 60. The deviation from a symmetricalprofile is given by a camber line 62, which is a median line through theairfoil profile 50. The median line can be found by drawing inscribedcircles from the leading edge 56 to the trailing edge 58. The medianline follows the centres of these inscribed circles and the deviation ordistance from the chord 60 is called the camber f. The asymmetry canalso be defined by use of parameters called the upper camber and lowercamber, which are defined as the distances from the chord 60 and thesuction side 54 and pressure side 52, respectively.

Airfoil profiles are often characterised by the following parameters:the chord length c, the maximum camber f, the position d_(f) of themaximum camber f, the maximum airfoil thickness t, which is the largestdiameter of the inscribed circles along the median camber line 62, theposition d_(t) of the maximum thickness t, and a nose radius (notshown). These parameters are typically defined as ratios to the chordlength c.

FIG. 4 shows a schematic view of flow velocities and aerodynamic forcesat the airfoil profile 50. The airfoil profile is located at the radialposition or radius r of the rotor of which the blade is part, and theprofile is set to a given twist or pitch angle θ. An axial free streamvelocity v_(a), which according to theory optimally is given as ⅔ of thewind velocity v_(w), and a tangential velocity r·ω, which is oriented ina direction of rotation 64 for the rotor, combined form a resultantvelocity v_(r). Together with the chord line 60, the resultant velocityv_(r) defines an inflow angle, φ, from which an angle of attack α can bededucted.

When the airfoil profile 50 is impacted by an incident airflow, a lift66 is generated perpendicular to the resultant velocity v_(r). At thesame time, the airfoil is affected by a drag 68 oriented in thedirection of the resultant velocity v_(r). Knowing the lift and drag foreach radial position makes it possible to calculate the distribution ofresultant aerodynamic forces 70 along the entire length of the blade.These aerodynamic forces 70 are typically divided into two components,viz. a tangential force 74 distribution (in the rotational plane of therotor) and a thrust 72 oriented in a right angle to the tangential force74. Further, the airfoil is affected by a moment coefficient 75.

The driving torque of the rotor can be calculated by integrating thetangential force 74 over the entire radial length of the blade. Thedriving torque together with the rotational velocity of the rotorprovides the overall rotor power for the wind turbine. Integrating thelocal thrust 72 over the entire length of the blade yields the totalrotor thrust, e.g. in relation to the tower.

In the following (section 1), blade design according to conventionaldesign methods is described.

1 STATE OF THE ART BLADE DESIGN FOR WIND TURBINES

The rotor design for wind turbines of today is a compromise betweenaerodynamic performance and overall wind turbine design loads. Mostoften, the blade is designed for minimum cost of energy (COE) findingthe optimum trade-off between energy yield and turbine loads. This meansthat the aerodynamic design cannot be looked at as an isolated problem,because it does not make sense to look isolated at maximum energy yieldin the event that this may lead to excessive loading. Therefore,classical analytical or semi-analytical methods for designing the bladedo not sufficiently apply.

1.1 Blade Design Parameters

The aerodynamic design of new blade for a rotor directly involves thefollowing overall rotor radius, R, and the number of blades, B.

The overall blade planform, which is described via the followingparameters in FIG. 3 and FIG. 4: the chord length, c, twist, Θ andthickness t relative to chord c. These should all be determined as afunction of the local blade radius r.

The location of the pitch axis versus blade radius may be defined asx/c(r) and y/c(r), i.e. back-sweep and pre-bending. When the blade ismounted on the rotor, the pre-bending is a pre-deflection of the bladein the direction perpendicular to the rotor plane. The purpose ofpre-bending is to prevent the blade from hitting the tower when theblade is deflected during operation. The prescribed back-sweep allowsthe placement of the airfoil sections along the length axis of theblade, which influences the section loads throughout the blade.

One important key element in state of the art aerodynamic rotor designmethods is the use of pre-designed airfoils. Airfoils are selected forblade stations along the blade radius. The parameters describing eachairfoil section are shown in FIG. 4: The lift coefficient 66, c_(l), thedrag coefficient 68, c_(d), the moment coefficient 75, c_(m). Forindividual blade stations, these airfoil characteristics are alldescribed versus the angle of attack, α, which is then determined by theoverall blade inflow angle for every section.

The large operational range for wind turbine rotors and the need forrobust and reliable aerodynamic characteristics in all terrainconditions make wind turbine airfoils differ from traditional aviationand glider airfoils.

1.2 Control Strategy

As the receptor of the power and most of the loading, the blades on thewind turbine rotor are very important components in the wind turbinesystem design. Wind turbine blades are therefore designed with closeknowledge of the wind turbine control strategy. The control strategydefines how the rotor power is optimized and controlled for differentwind speeds.

Three fundamentally different control schemes exist:

-   -   1. Variable rotor speed where the design target point of the        rotor may be obtained for the wind speeds where the rotor speed        is variable. Usually blade pitch is kept constant.    -   2. Constant rotor speed with variable blade pitch. Here the        design target point of the rotor is approached as much as        possible by adjusting the blade pitch.    -   3. Constant rotor speed and constant blade pitch. Here the        design target point of the rotor can only be met at a single        wind speed.

FIGS. 6 a-6 c show the power characteristics for a typical variablespeed and pitch controlled (PRVS) wind turbine:

FIG. 6 a shows a typical power curve versus wind speed. At low windspeeds, the power increases with the wind speed until the rated power isreached. There are two important wind speed regions, viz. a poweroptimisation region and a power control region. Power is optimized inthe region, where the wind velocity is lower than a threshold valueillustrated by the dashed line in FIG. 6, whereas the power controlregion is found in the region, where the power is constant at higherwind velocities. During the power optimisation region, the rotor designtarget point is tracked by varying either blade tip pitch or rotorspeed. This is done to maximize power and thereby energy yield.

FIGS. 6 b and 6 c show the control parameters that govern the windturbine blade design: FIG. 6 b shows the rotor speed, Ω, versus windspeed and FIG. 6 c shows the blade tip pitch angle, Θ. The rotor speedhas a minimum value at low wind speeds and when optimum power is trackeduntil rated power this corresponds to a linear increase in rotor speedwith wind speed. When reaching a given maximum value for the rotorspeed, this is then kept constant during power control. The blade pitchis typically kept constant during power optimization and is thenincreasing with wind speed during power control to prevent the powerfrom exceeding the rated value.

During the power control region, for most turbines the power is keptclose to the drive train rated power by adjusting the blade pitch angleeither so that the blade goes into stall or oppositely towards lessloading. Some turbines have stall control, where blade pitch is keptconstant. Here the rated power value is obtained by letting parts of theblade go into stall passively by design.

1.3 Rotor Design Target Point

Independently on the type of power optimisation, a wind turbine blade isdesigned for operation at one design target point. For variable rotorspeed and/or variable blade pitch, operation at the design target pointmay be obtained within a wind speed range, whereas for a stallcontrolled rotor, operation at the design target point appears only at asingle wind speed.

The rotor design target point is characterised by the correspondingdesign tip speed ratio, defined as the ratio between the tip speed andthe wind speed, X=r·Ω/V, where Ω is the rotational speed of the rotor.At the design target point, the rotor power coefficient is maximumcompared to operating points away from the design target point.

The rotor design point may be seen as an average over the entirelongitudinal extent of the first longitudinal segment, or it may be seenas an individual target for a plurality of smaller radial segmentswithin the first longitudinal segment. Yet again, it may be seen as anindividual target for each cross-section of the first longitudinalsegment of the blade.

When the rotor design target point is determined and the turbine controlstrategy is settled, airfoils are selected and the rotor radius andnumber of blades are decided upon. The parameters that are left are thenthe local chord, twist and thickness versus blade radius plus the localsection design target point. These are then found by optimizing therotor design target point performance taking into account loads and costof energy. The rotor power coefficient at the design target point istherefore not necessarily the optimum achievable value, but for a givenrotor there will always exist one design target point.

1.4 Local Section Design Target Point

The local section design target point is defined from the velocitytriangle for the given section as shown in FIG. 7. Here, the resultingvelocity, W, is composed by the axial flow speed, V(1−a), and thetangential flow speed, r·Ω(1+a′). The tangent to the overall flow angle,φ, is equal to the ratio between the axial component and the tangentialcomponent. The axial induction factor, a, expresses the percentagereduction of the free flow speed at the rotor plane. The tangentialinduction factor expresses the percentage rotation of the rotor wake inthe rotor plane caused by the rotor. The overall flow angle, φ, is againcomposed by the local twist angle, Θ, and the local angle of attack, α,as shown in FIG. 4.

When knowing the local chord c, and local twist Θ as well as the airfoilsection force coefficients versus the local angle of attack α, it ispossible to use the so called blade element momentum method (BEM) tosolve for the equilibrium between the overall gross flow through therotor annulus covered by the blade section and the local forces on eachof the blades. The resulting normal force perpendicular to the rotorplane and the tangential force parallel to the rotor plane may becalculated. Through this calculation procedure, the induction factorsare determined and when operating at the rotor design target point, theinduction factor is then denoted as the target induction factor.

Vice versa, if deciding on the target induction factor it is possible toderive the local chord and twist when knowing the airfoil section. Inthe event of designing the local section for optimum aerodynamicperformance, it can be shown that the optimum axial induction factorapproaches ⅓ for high values of the tip speed ratio, whereas thetangential induction factor approaches zero.

A simple method exists for determining the exact optimum induction andthereafter local chord and twist for optimum aerodynamic performance. Anexample of such a method is the method by Glauert published with the BEMmethod (Glauert, H. Airplane propellers in Aerodynamic Theory ed.Durand, W. F. Dower Publications, Inc. New York).

1.5 Classical Aerodynamic Blade Design

The classical blade element momentum (BEM) theory by Glauert allowssolving the rotor flow by simple means by establishing the equilibriumbetween the overall flow through the rotor disc and the local flowaround the airfoils on the blades by dividing the rotor disk intoannular stream tubes. If drag is neglected a simple expression can thenbe found for the optimum rotor:

${{\frac{B}{R}{X\left( {cC}_{I} \right)}} = {8\pi\; x\frac{a\;}{{1 - a}\;}\frac{\sin^{2}\Phi}{\cos\;\Phi}}},$where as previously mentioned, x=Ω·r/V is the tip speed ratio, V is thedesign point wind speed, X is the tip speed based on R and Φ is thelocal inflow angle, and a is the axial induction.

When defining an aerodynamic blade shape the first step is to choose thenumber of blades and the design tip speed ratio. The ideal rotor loadingdefined as the chord length multiplied by the lift coefficient (c·c_(l))and the inflow angle, Φ, can then be found versus radius. On basis ofthe ideal rotor loading, the target loading may be decided on takinginto account loads and practical limitations.

Next, selecting the airfoils for the individual blade sections andknowing the flow angle makes it possible to decide the blade twist. Thisis commonly chosen so that the airfoil lift-to-drag ratio is optimal onas large a part of the blade as possible to maximize the rotor powercoefficient. The blade operating lift coefficient c_(l,o) is thentypically the airfoil design lift coefficient c_(ld) and the chord canthen be derived from the target loading. However on parts of the bladethere will be a difference between the blade operating lift coefficientc_(l,o) versus the airfoil design lift coefficient c_(ld) due toconsiderations on loads, manufacture, etc. When there is a difference,the operating lift coefficient will not lead to optimum lift-to-drag asindicated in FIG. 16 and the operating angle of attack, α_(o) will notbe equal to the airfoil design angle of attack, α_(d).

The blade thickness is chosen as a compromise between structural andaerodynamic considerations, since higher thickness favours the bladestructure at the expense of degeneration of the airfoil lift-drag ratio.

The BEM method also reveals that an axial induction of ⅓ unfortunatelyis associated with a high thrust force on the rotor and that thrust andthereby loads can be reduced significantly with only little reduction inrotor power. This is because designing for the aerodynamic optimum in asingle point does not take into account off-design operation nor loadsand thereby minimum cost of energy. To reduce loads the axial inductionis often reduced compared to the optimum value of ⅓. On the other handwhen including also off-design operation in the design problem, such asoperation close to power control, there may be a required minimum valuefor the target induction factor to prevent premature stall on the bladesleading to unnecessary noise and power loss. Hence, for a modern rotorthe target induction factor is not necessarily identical to theaerodynamic optimum induction and there is not a single optimum for thetarget induction versus blade radius, since such an optimum depends onnumerous factors.

In FIGS. 8 a and 8 b the ideal values (dashed lines) for loading(c·C_(l)) and inflow angle, Φ, are shown together with the real targetvalues (full drawn lines) for inflow and loading of a typical windturbine blade. It can be seen that there is a nearby match between thetwo curves over a large part of the blade but that there are alsodiscrepancies. It is seen that the target value especially deviates fromthe ideal values for low values of r, i.e. near the blade root. This ismainly due to structural considerations as explained in relation toFIGS. 2 and 5. Furthermore, it appears clearly from FIG. 8 a and FIG. 8b that since loading and inflow angle varies non-linearly with bladeradius, this will also be the case for both the chord and twist—not onlyfor the ideal blade but also for a typical commercial blade.

1.6 Blade Regions

In accordance with blade design, a blade may be divided into fourdifferent regions as shown in FIGS. 2 and 5:

-   1. The blade root region 30 next to the hub, which is predominantly    circular.-   2. The transition region 32 between the blade root region and the    remaining blade part.-   3. An airfoil shaped part 34′, which is the main part of the blade.    Typically the airfoil shaped part extends from the area of the blade    with maximum chord and towards the blade tip part.-   4. A blade tip part 36—usually less than the outer 10% of the blade.

The blade root region 30 is the interface from the blade to the bladebearing and the hub, and therefore this region has to end in a circularflange. The design is therefore mainly structural. The blade chord andthickness in this region correspond to the root flange diameter and thetwist cannot be defined in this region. Due to the poor aerodynamiccharacteristics of a circular section, the resulting normal forcecomponent will be significantly too small to balance the rotor flow, theinduction will be too small and the inflow angle will be too highleading to a poor local power coefficient.

The transition region 32 is formed by the morphing from the airfoilshaped part 34′ to the blade root region 30. Chord, twist and thicknessmorph to their respective values at the beginning of the airfoil shapedpart 34′. Note that the morphing is not necessarily linearly dependenton blade radius. In this region, the clean sectional characteristics arepoor due to the high relative thickness and the local chord is notsufficiently high to obtain the right normal force component. However,this may be altered if flow control is used to achieve the rightcombination of the normal force component and local inflow to yieldmaximum possible power coefficient.

The airfoil shaped part 34′ is designed primarily from aerodynamicreasons, opposed to the other regions 30, 32, 36. The airfoil shapedpart 34′ is the largest part of the blade and it is this part, which ismainly responsible for both the rotor power and the turbine loads. Thisregion is designed with near-optimum blade aerodynamics taking intoaccount off-design operation and loads.

The blade tip part 36 is the very tip region, which is designed mainlyfor noise and load concerns and the optimum values of chord and twistmay therefore be deviated from. The chord and thickness go to zerotowards the blade tip, whereas the twist ends at a finite value.

1.7 Summary

Traditionally modern wind turbine blades are designed by initiallydesigning the outer shape and the aerodynamic performance of the bladeitself in order to obtain the target loading and target axial inductionfor each radial section of the airfoil section of the blade. Firstafterwards, it is determined how to plan the manufacturing in accordancewith the design specification for the blade.

2 TRANSFORMABLE BLADES

The present invention provides a departure from the traditional processof designing modern wind turbine blades. The invention provides a newdesign process, in which the production is optimised in relation toeffective methods of manufacturing a base part or main blade part of awind turbine blade, and where the base part of the blade is retrofittedwith flow guiding devices in order to obtain the proper aerodynamicspecifications, i.e. to obtain the target axial induction factor andloading for each radial section. Thus, the base part of the blade maydeviate substantially from the target design point and the optimumaerodynamic design.

The invention primarily relates to a different design of the airfoilshaped part 34′ of the blade, see FIG. 5. In the following, bladesaccording to the invention will sometimes be referred to astransformable blades.

FIG. 9 shows a first embodiment of a wind turbine blade according to theinvention. Similar to the conventional method of designing a windturbine blade, the blade is divided into a root region 130, a transitionregion 132, and an airfoil region 134. The airfoil region comprises ablade tip part 136 and a first longitudinal section 140 of the blade.The first longitudinal section of the blade is divided into a first basepart 141 and a number of flow altering devices 146-149. The first basepart 141 has a profile, which has a simplified structure with respect tofor instance modularity of blade parts and/or manufacturing of the firstbase part 141, and which at the rotor design point in itself deviatessubstantially from the target axial induction factor and/or the targetloading. Therefore, the first longitudinal section 140 is provided withthe flow altering devices, which are here depicted as a first slat 146and a second slat 147, as well as a first flap 148 and a second flap149. Although the use of such flow altering devices is highlyadvantageous in order to obtain the target axial induction factor andthe target loading, the invention is not restricted to such flowaltering devices only. The first longitudinal section 140 extends alongat least 20% of the longitudinal length of the airfoil region 134.

Further, the blade may be provided with flow altering devices arrangedat the transition region 132 and possibly the root region 130 of theblade, here depicted as a slat 133.

In the first embodiment, the airfoil shaped part of the airfoil regionis replaced by a single longitudinal section comprising a base part andflow altering devices. However, the airfoil shaped part may be dividedinto additional longitudinal sections as shown in FIGS. 10 and 11.

FIG. 10 shows a second embodiment of a wind turbine blade according tothe invention. Similar to the conventional method of designing a windturbine blade, the blade is divided into a root region 230, a transitionregion 232, and an airfoil region 234. The airfoil region comprises ablade tip part 236, a first longitudinal section 240, and a secondlongitudinal section 242. The first longitudinal section 240 of theblade is divided into a first base part 241 and a number of first flowaltering devices 246. The second longitudinal section 242 of the bladeis divided into a second base part 243 and a number of second flowaltering devices 248. The first base part 241 and the second base part243 have profiles, which have a simplified structure with respect to forinstance modularity of blade parts and/or manufacturing of the baseparts 241, 243, and which at the rotor design point in itself deviatessubstantially from the target axial induction factor and/or the targetloading. Therefore, the longitudinal sections are provided with the flowaltering devices, which are here depicted as a first slat 246 and afirst flap 248, however; the flow altering devices are not restricted tosuch flow altering devices only. The first longitudinal section 240 andthe second longitudinal section 242 both extend along at least 20% ofthe longitudinal length of the airfoil region 234.

Further, the blade may be provided with flow altering devices arrangedat the transition region 232 and possibly the root region 230 of theblade, here depicted as a slat 233.

FIG. 11 shows a third embodiment of a wind turbine blade according tothe invention, wherein like reference numerals refer to like parts ofthe second embodiment shown in FIG. 10. Therefore, only the differencebetween the two figures is explained. The third embodiment differs fromthe second embodiment in that the airfoil region 334 further comprises atransition section 344 arranged between the first longitudinal section340 and the second longitudinal section 342. The transition section 344comprises a transition base part 345, which is formed by morphing fromthe end profiles of the first base part 341 and second base part 343,respectively. Accordingly, the transition base part 345 also has aprofile shape, which at the rotor design point in itself deviatessubstantially from the target axial induction factor and/or the targetloading. Consequently, the transition section 344 is also provided witha number of flow altering devices 346, 348.

Further, the blade may be provided with flow altering devices arrangedat the transition region 332 and possibly the root region 330 of theblade, here depicted as a slat 333.

It will be apparent from the later description that the embodimentsshown in FIGS. 9-11 also may be provided with surface mounted elements,vortex generators and the like.

2.1 Local Sub-Optimum Twist and/or Chord Length

When designing transformable blades having simplified base parts, twoprofile characteristics of the base parts will typically differ locallyfrom the optimum target condition, viz. the local blade twist andthereby the inflow angle at the rotor design point, and the local chordlength.

FIG. 12 shows a first situation illustrating the above mentioneddeviation. As explained in section 1.4, the local section design targetpoint is defined from the velocity vector triangle, where the resultingvelocity v_(inflow) is composed by the axial flow speed,V_(wind)(1−a_(target)), and the tangential flow speed, r·ω(1+a′), seealso FIG. 12 a. This condition is only met at the rotor design point,when the inflow angle is Φ₁. At the rotor design point, the localsection has an operational lift coefficient c_(l) and an operationaldrag coefficient c_(d). The resultant aerodynamic forces may aspreviously explained be divided into a tangential force T, which isoriented in the rotational plane of the rotor and a loading or thrust,which is the normal force N_(target) oriented normally to the rotorplane 64′.

For the given profile of the section having a given local chord lengthc, the target conditions for achieving the target axial induction factora_(target) and the target normal load N_(target) for the local bladeprofile at the rotor design point is met only, when the local twistangle is equal to a target twist θ₁ and the angle of attack is equal toα₁.

However, the local blade profile for the base part has an actual twistθ₂, which is lower than the target twist θ₁. Consequently, the inflowangle is shifted to an altered angle Φ₂, which is lower than Φ₁.Furthermore, the angle of attack is changed to an altered angle ofattack α₂, which is larger than α₁. Consequently, the two shown vectortriangles are as shown in FIG. 12 b shifted and the blade sectionobtains an inflow condition having an altered resultant velocity vectorv_(inflow2), at which an actual axial induction factor a₂ becomes largerthan the target axial induction factor a_(target). Further, the liftcoefficient is shifted to an altered lift coefficient c_(l2), and analtered drag coefficient c_(d2). Consequently, the normal load isshifted to an actual normal load N₂, which is larger than the targetnormal load N_(target). Consequently flow altering devices are needed inorder compensate for the altered inflow conditions and normal load.

In order to obtain the target axial induction factor a_(target), theinflow angle must be shifted back to Φ₁ as shown in FIG. 12 c.Consequently, the compensated angle of attack must equal α₃=Φ₁−θ₂. Atthis angle of attack, the flow altering devices (here depicted as aflap) must alter the drag coefficient and lift coefficient to alteredvalues c_(d3) and c_(l3), at which the resultant normal load becomesequal to the target normal load N_(target). Thus, the flow alteringdevices are used to reduce the axial induction factor from a₂ toa_(target), and reduce the load from N₂ to N_(target).

FIG. 13 shows a similar situation, but where the local blade profile forthe base part having a given local chord length c has an actual twistθ₂, which is higher than the target twist θ₁ (as shown in FIG. 13 b).Consequently, the inflow angle is shifted to an altered angle Φ₂, whichis larger than Φ₁. Furthermore, the angle of attack is changed to analtered angle of attack α₂, which is smaller than α₁. Consequently, thetwo shown vector triangles are as shown in FIG. 12 b shifted and theblade section obtains an inflow condition having an altered resultantvelocity vector v_(inflow2), at which an actual axial induction factora₂ becomes smaller than the target axial induction factor a_(target).Further, the lift coefficient is shifted to an altered lift coefficientc_(l2), and an altered drag coefficient c_(d2). Consequently, the normalload is shifted to an actual normal load N₂, which is smaller than thetarget normal load N_(target). Consequently flow altering devices areneeded in order to compensate for the altered inflow conditions andnormal load.

In order to obtain the target axial induction factor a_(target), theinflow angle must be shifted back to Φ₁ as shown in FIGS. 13 a and 13 c.Consequently, the compensated angle of attack must equal α₃=Φ₁−θ₂. Atthis angle of attack, the flow altering devices (here depicted as aflap) must alter the drag coefficient and lift coefficient to alteredvalues c_(d3) and c_(l3), at which the resultant normal load becomesequal to the target normal load N_(target). Thus, the flow alteringdevices are used to increase the axial induction factor from a₂ toa_(target), and increase the load from N₂ to N_(target).

FIG. 14 yet again shows a similar situation. For the given profile ofthe section having a given pitch angle θ₁ and angle of attack α₁, thetarget conditions for achieving the target axial induction factora_(target) target normal and load N_(target) for the local blade profileat the rotor design point are only met, when the chord length is equalto a target chord c₁ (as shown in FIG. 14 a).

However, the local blade profile for the base part has an actual chordlength c₂, which is lower than the target chord c₁. Consequently, theinflow angle is shifted to an altered angle Φ₂, which is higher than Φ₁.Furthermore, the angle of attack is changed to an altered angle ofattack α₂, which is larger than α₁. Consequently, the two shown vectortriangles are as shown in FIG. 14 b shifted and the blade sectionobtains an inflow condition having an altered resultant velocity vectorv_(inflow2), at which an actual axial induction factor a₂ becomessmaller than the target axial induction factor a_(target). Further, thelift coefficient is shifted to an altered lift coefficient c_(l2), andan altered drag coefficient c_(d2). Consequently, the normal load isshifted to an actual normal load N₂, which is smaller than the targetnormal load N_(target). Consequently flow altering devices are needed inorder compensate for the altered inflow conditions and normal load.

In order to obtain the target axial induction factor a_(target), theinflow angle must be shifted back to Φ₁ as shown in FIG. 14 c.Consequently, the compensated angle of attack must equal α₃=Φ₁−θ₂. Atthis angle of attack, the flow altering devices (here depicted as aflap) must alter the drag coefficient and lift coefficient to alteredvalues c_(d3) and c_(l3), at which the resultant normal load becomesequal to the target normal load N_(target). Thus, the flow alteringdevices are used to increase the axial induction factor from a₂ toa_(target), and increase the load from N₂ to N_(target).

FIG. 15 shows yet again a similar situation, but where the local bladeprofile for the base part having a given twist angle θ₂ has an actualchord c₂, which is larger than the target chord c₁ (as shown in FIG. 15b). Consequently, the inflow angle is shifted to an altered angle Φ₂,which is lower than Φ₁. Furthermore, the angle of attack is changed toan altered angle of attack α₂, which is lower than α₁. Consequently, thetwo shown vector triangles are as shown in FIG. 15 b shifted and theblade section obtains an inflow condition having an altered resultantvelocity vector v_(inflow2), at which an actual axial induction factora₂ becomes larger than the target axial induction factor a_(target).Further, the lift coefficient is shifted to an altered lift coefficientc_(l2), and an altered drag coefficient c_(d2). Consequently, the normalload is shifted to an actual normal load N₂, which is larger than thetarget normal load N_(target). Consequently flow altering devices areneeded in order to compensate for the altered inflow conditions andnormal load.

In order to obtain the target axial induction factor a_(target), theinflow angle must be shifted back to Φ₁ as shown in FIGS. 15 a and 15 c.Consequently, the compensated angle of attack must equal α₃=Φ₁−θ₂. Atthis angle of attack, the flow altering devices (here depicted as aflap) must alter the drag coefficient and lift coefficient to alteredvalues c_(d3) and c_(l3), at which the resultant normal load becomesequal to the target normal load N_(target). Thus, the flow alteringdevices are used to decrease the axial induction factor from a₂ toa_(target), and decrease the load from N₂ to N_(target).

2.2 Flow Altering Devices and Aerodynamic Effect

In this subsection various flow altering devices, which can be used tocompensate for the off-target twist and chord, are described togetherwith their aerodynamic effect. In general multi-element devices, such asflaps and/or slats, as shown in FIG. 19, or surface mounted elements asshown in FIG. 18 are needed in order to compensate for the substantialdeviation from target twist and chord of the base part of thelongitudinal sections of the blade. However, it may be necessary to useadditional flow altering devices, such as high lift devices, e.g. vortexgenerators and/or Gurney flaps, in order to obtain the correct lift anddrag coefficients at the given angle of attack.

FIG. 17 shows a first example of flow altering devices 80 forcompensating for off-target design parameters of the base part of therespective longitudinal section of the blade. In this embodiment, theflow altering means consists of a number of ventilation holes 80 forblowing or suction between an interior of the blade and an exterior ofthe blade. The ventilation holes are advantageously applied to thesuction side of the blade as shown in FIGS. 17 a and 17 b. Theventilation holes 80 can be utilised to create a belt of attached flow.Air vented from the ventilation holes 80 may be used to energise andreenergise the boundary layer in order to maintain the flow attached tothe exterior surface of the blade as shown in FIG. 17 b. Alternatively,the ventilation holes may be used for suction as shown in FIG. 17 a,whereby the low momentum flow in the boundary layer is removed and theremaining flow thereby reenergised and drawn towards the surface of theblade. Alternatively, the ventilation holes may be used to generate apulsating flow, e.g. as a synthetic jet. Despite not generating a flow,this transfers momentum to the flow and thereby reenergises the boundarylayer and alters flow separation. Examples of such embodiments are shownin FIGS. 17 c and 17 d, in which the ventilation holes are provided withmembranes. The membranes may be provided near the exterior surface ofthe blade as shown in FIG. 17 d or near the interior surface of theblade as shown in FIG. 17 c.

The full drawn line in FIG. 17 e shows the relationship between the liftcoefficient and the inflow angle (or alternatively the angle of attack)for the basic airfoil without suction or blowing. By using suction asshown in FIG. 17 a or tangential blowing, i.e. venting air substantiallytangentially to the surface of the blade, the boundary layer isenergised and reenergised. Likewise a pulsating jet as shown in FIGS. 17c and 17 d will energise the boundary layer. Consequently, the liftcoefficient becomes larger. At the same time, the maximum liftcoefficient is found at a slightly higher inflow angle. Thus, the graphis shifted up and slightly to the right as shown with the dashed line inFIG. 17 e. Alternatively, it is possible to apply blowing in anoff-tangential angle, e.g. at an angle of more than 45 degrees andpossibly substantially normally to the blade surface. In this case, theboundary layer becomes detached from the surface of the blade.Consequently, the lift coefficient becomes lower. At the same time, themaximum lift coefficient is found at a slightly lower inflow angle.Thus, the graph is shifted down and slightly to the left as shown withthe dotted line in FIG. 17 e.

FIG. 18 shows a second example of flow altering devices of flow alteringdevices 180, 181, 182 for compensating for off-target design parametersof the base part of the blade. In this embodiment, the flow alteringmeans consists of a number of surface mounted elements. FIG. 18 a showsa first embodiment, in which a first trailing edge element 180 ismounted near the trailing edge of the blade on the suction side of theblade, a second trailing edge element 181 is mounted near the trailingedge of the blade on the pressure side of the blade, and a leading edgeelement 182 is mounted near the leading edge of the blade on thepressure side of the blade. FIG. 18 b shows a second embodiment, inwhich only a first trailing edge element is utilised on the suction sideof the blade.

The full drawn line in FIG. 18 c shows the relationship between the liftcoefficient and the inflow angle (or alternatively the angle of attack)for the basic airfoil without the use of surface mounted element. Byutilising the leading edge element 182 and the second trailing edgeelement 181 on the pressure side of the blade, the effective camber ofthe airfoil is increased and the operating lift coefficient at the rotordesign point is increased. The maximum lift coefficient is alsoincreased. By utilising the first trailing edge element 180 on thesuction side of the blade, the camber of the airfoil is reduced and theoperating lift coefficient at the rotor design point as well as themaximum lift coefficient is decreased.

FIG. 19 shows the effect of using multi-element airfoils, such as slatsor flaps, as flow guiding devices. The depicted graph shows therelationship between the lift coefficient and the inflow angle (oralternatively the angle of attack) for the basic airfoil without the useof multi-element airfoils. By utilising a trailing edge flap orientedtowards the pressure side of the blade, the graph is shifted towardslower angles of attack. By utilising a trailing edge flap orientedtowards the suction side of the blade, the graph is shifted towardshigher angles of attack. By using a slat near the suction side of theblade, the lift coefficient is increased, and further the maximum liftcoefficient is found at a slightly higher angle of attack. By using aslat near the suction side of the blade and a flap oriented towards thepressure side of the blade, the lift coefficient is increased, andfurther the maximum lift coefficient is found at a lower angle ofattack. By using a slat near the suction side of the blade and a flaporiented towards the suction side of the blade, the lift coefficient isincreased, and further the maximum lift coefficient is found at a higherangle of attack.

Slats and flaps may be implemented in various ways. A slat may forinstance be connected to the first base part of the blade via aconnection element as shown in FIG. 19 b. The slat may be connected tothe first base part in such a way that it is rotational and/ortranslational movable in relation to the first base part. Likewise aflap may be provided as a separate element as shown in FIG. 19 c, whichmay be moved rotational and/or translational in relation to the firstbase part. Thus, the blade profile is a multi element profile.Alternatively, the flap may be implemented as a camber flap as shown inFIG. 19 d, which can be used to change the camber line of the bladeprofile.

FIG. 20 shows another example of flow altering devices 280 forcompensating for off-target design parameters of the base part of theblade. In this embodiment, the flow altering means comprises a deviceattached to the pressure side at the trailing edge, in this case aGurney flap 280 as shown in FIG. 20 a. Other attachments with similarflow altering means are a triangular wedge or a rip forming an angle ofmore than 90 degrees with the surface of the profile. The full drawnline in FIG. 20 b shows the relationship between the lift coefficientand the inflow angle (or alternatively the angle of attack) for thebasic airfoil without the use of a surface mounted element. By utilisingthe Gurney flap, the operating lift coefficient at the rotor designpoint is increased as well as the maximum lift coefficient, which isalso increased as shown as the dashed line in FIG. 20.

FIG. 21 shows yet another example of flow altering devices 380 forcompensating for off-target design parameters of the base part of theblade. In this embodiment, the flow altering means comprises a number ofvortex generators as shown in FIG. 21 a. The vortex generators 380 arehere depicted as being of the vane type, but may be any other type ofvortex generators. The vortex generators 380 generate coherent turbulentstructures, i.e. vortices propagating at the surface of the bladetowards the trailing edge of the blade. The vortex generatorsefficiently change the optimum angle of attack for the radial sectionand alter the lift of the blade section by reenergising the boundarylayer and delaying separation.

FIG. 21 b shows an advantageous embodiment having an arrangement ofpairs of vane vortex generators, which has shown to be particularlysuited for delaying separation of airflow. The arrangement consists ofat least a first pair of vane vortex generators comprising a first vaneand a second vane, and a second pair of vane vortex generatorscomprising a first vane and a second vane. The vanes are designed astriangular shaped planar elements protruding from the surface of theblade and are arranged so that the height of the vanes increases towardsthe trailing edge of the blade. The vanes have a maximum height h, whichlies in an interval of between 0.5% and 1% of the chord length at thevane pair arrangement. The vanes are arranged in an angle b of between15 and 25 degrees to the transverse direction of the blade. Typicallythe angle b is approximately 20 degrees. The vanes of a vane pair arearranged so that the end points, i.e. the point nearest the trailingedge of the blade, are spaced with a spacing s in an interval of 2.5 to3.5 times the maximum height, typically approximately three times themaximum height (s=3 h). The vanes have a length l corresponding tobetween 1.5 and 2.5 times the maximum height h of the vanes, typicallyapproximately two times the maximum height (l=2 h). The vane pairs arearranged with a radial or longitudinal spacing z corresponding tobetween 4 and 6 times the maximum height h of the vanes, typicallyapproximately five times the maximum height (z=5 h). The full drawn linein FIG. 21 c shows the relationship between the lift coefficient and theinflow angle (or alternatively the angle of attack) for the basicairfoil without the use of vortex generators. By utilising the vortexgenerators 380, the maximum lift coefficient is shifted towards a higherangle of attack. The dotted line shows the corresponding relationship,when vortex generators are positioned in a forward position, i.e.towards the leading edge of the blade, whereas the dashed line shows thecorresponding relationship, when vortex generators are positioned in abackward position, i.e. towards the trailing edge of the blade. It isreadily seen that the use of vortex generators can be used to change thedesign inflow angle as well as the maximum lift.

FIG. 22 shows yet another example of flow altering devices 480 forcompensating for off-target design parameters of the base part of theblade. In this embodiment, the flow altering means comprises a spoilerelement, which protrudes from the pressure side of the blade as shown inFIG. 22 a. The spoiler element is usually used at the transition regionof the blade and possibly at an inboard part of the airfoil region ofthe blade. The full drawn line in FIG. 22 b shows the relationshipbetween the lift coefficient and the inflow angle (or alternatively theangle of attack) for the basic airfoil without the use of surfacemounted element. It is seen that the lift coefficient is very low forthe transition region. By utilising a spoiler element, the maximum liftcoefficient is increased significantly. By utilising a spoiler elementpositioned at a forward position of the blade, i.e. towards the leadingedge of the blade or towards the position of maximum thickness, theoperating lift coefficient at the rotor design point is increased aswell as the maximum lift coefficient as shown with dashed line in FIG.22 b. By utilising a spoiler element positioned at a backward positionof the blade, i.e. towards the trailing edge of the blade or towards theposition of maximum thickness, the operating lift coefficient at therotor design point as well as the maximum lift coefficient is shiftedtowards a higher value as well as towards a higher angle of attack asshown with dotted line in FIG. 22 b.

3 SIMPLIFIED BASE PARTS OF BLADE

In this section, a number of simplified base part structures fortransformable blades are described.

3.1 Base Part with Sub-Optimum Twist

A modern wind turbine blade designed according to conventional methodswill have an inherent twist, which is non-linearly dependent on thelocal radius of the blade. Furthermore, the twist is relatively high—asmuch as 20 degrees. The twist especially needs to be quite large at theinboard part of the airfoil region and the transition region of theblade, since the resulting inflow velocity at the rotor design pointchanges relatively much in the radial direction of the blade in thispart, whereas the twist is quite small in the outboard part of the bladenear the blade tip, since the resulting inflow velocity at the rotordesign point in this part of the blade changes slower in the radialdirection of the blade. Due to this non-linearity, the design of modernwind turbine blades according to conventional methods is quite complex.Accordingly, the design of mould parts for manufacturing such bladeswill also be quite complex.

Therefore, from a design and manufacturing point of view it would beadvantageous to obtain a base part of the blade having a simplifiedtwist, such as a linearly dependent twist or a reduced twist compared toan optimum twist. Such simplified twist makes it feasible to achieve amodular blade design, in which the base part with the non-optimum twistcan be used for several different blade types and blade lengths. Thus,it is possible to reuse the base part of an existing blade furtheroutboard on a larger/longer blade, or alternatively reuse the base partof an existing blade further inboard on a smaller/shorter blade. All inall, it is possible to make a blade design in such a way that the bladedesign of the airfoil region is put together from pre-designed sectionsand that blades of different lengths can be composed partly fromsections already existing from previous blades.

However, putting such a constraint on the blade design implies the needfor using flow altering devices in order to compensate for not beingable to operate at the target design ideal twist for the different bladesections as explained in subsection 2.1.

In order to compensate for this non-ideal twist, a blade may be dividedinto a number of separate radial blade sections 38 as shown in FIG. 2,which each are provided with flow altering devices (not shown) in orderto compensate for the non-ideal twist for that radial blade section 38.The radial blade sections 38 are here depicted as extending onlyslightly into the airfoil area 34. However, in order to obtain anoptimum compensation for the non-optimum twist, the blade must beprovided with individually compensated radial blade sections 38 alongsubstantially the entire airfoil area 34. Since the twist of the outerpart of the blade, i.e. near the tip, is small, not all embodiments ofthe blade according to the invention need to be provided with flowaltering devices near the tip end. However, preferably at least theinner 75% of the airfoil area 34 is provided with radial blade sections38 having flow altering devices.

Each of the radial sections 38 has an individual average angle of attackfor a given design point and the base part of the blade has a sectionalairfoil shape, which without flow altering devices has a sectionaloptimum angle of attack. Flow altering devices, e.g. as shown in FIGS.17-22, may be used to shift the optimum angle of attack of the sectionalairfoil shape towards the average angle of attack for the radialsection.

FIG. 23 a shows graphs of an average angle of attack 78 for the blade asa function of the radial distance r from the hub of the rotor. FIG. 23 aalso shows a graph of an optimum angle of attack 76 of the blade withoutflow altering devices as a function of the radial distance from the hub.It can be seen that the average angle of attack 78 is higher than theoptimum angle of attack 76, which clearly indicates that the blade doesnot have an optimum blade twist. Therefore, the blade can be providedwith flow altering devices as for instance shown in FIGS. 17-22 in orderto shift the optimum angle of attack with a shift angle Δα, therebyshifting the optimum angle of attack towards the average angle of attackfor a given radial distance r from the hub.

FIG. 23 b shows a graph of the shift angle Δα as a function of theradial distance from the hub. It can be seen that the shift angle Δα iscontinuously decreasing with increasing distance r from the hub.

FIGS. 23 c and 23 d illustrate the effect of providing the blade withflow altering devices for the outer part 44 and the inner part 42 of theblade, respectively. FIG. 23 c shows graphs of the relationship betweenthe drag coefficient and the lift coefficient as well the relationshipbetween the angle of attack and the lift coefficient. The graphs areexamples of design parameters for the blade at a given radial distancefrom the hub falling within the outer part 44 of the blade. The designpoint is depicted with a dot and is chosen based on an optimumrelationship between the lift coefficient and the drag coefficient, e.g.by maximising the lift-to-drag ratio. By providing the blade with flowaltering devices, the graph showing the relationship between the liftcoefficient and the angle of attack is shifted towards higher angles,thereby compensating for the “missing” twist of the outer part 44 of theblade.

FIG. 23 d shows similar graphs for the inner part 42 of the blade. Itcan be seen that the use of flow altering devices on the inner part 42of the blade has two effects, viz. that the relationship between thelift coefficient and the angle of attack is shifted towards a higherangle of attack and towards a larger lift coefficient. Thus, the flowaltering devices not only compensate for the “missing” twist of theinner part 42 of the blade, but also compensate for the non-optimumprofile with respect to generating lift of the inner part 42 of theblade, which typically would comprise the root area 30 and thetransition area 32 of the blade.

However, in relation to modularity of blade parts, it is advantageousthat the twist is linearly dependent on the local blade radius. FIGS. 24a-d illustrate the relationship between twist, θ, and local radius forvarious embodiments having a linearly dependent twist. The dashed linesillustrate the optimum twist angle in order to obtain the rotor designtarget point for a blade without flow altering devices, and the fulldrawn lines illustrate the relationship between twist angle and localblade radius for a base part having sub-optimum twist and provided withflow altering devices. As shown in FIG. 24 a, the twist angle may belower than the optimum twist angle along the entire longitudinal extendof the longitudinal blade section. FIG. 24 b illustrates a secondembodiment, in which the twist angle of the base part is equal to theoptimum twist angle for a single cross-section only, and where theremainder of the longitudinal blade section has a twist angle, which islower than the optimum twist angle. However, in principle, thelongitudinal blade section may have one part, in which the twist angleis lower than the optimum twist angle, a second part, in which the twistangle is higher than the optimum twist angle, and a third part, in whichthe twist angle is higher than the optimum twist angle. This isillustrated in FIG. 24 c. Yet again, it is highly advantageous, if thebase part is designed without any twist, which is illustrated in FIG. 24d.

3.2 Base Part with Linear Chord

As previously mentioned, a modern wind turbine blade designed accordingto conventional methods has a chord length distribution, which isnon-linearly dependent on the local radius of the blade. However, aswith respect to twist, from a design and manufacturing point of view itis advantageous to obtain a base part of the blade having a simplifiedchord length distribution, e.g. a linearly chord length.

FIGS. 24 e-g illustrate the relationship between chord length, c, andlocal radius for various embodiments having a linearly dependent chordlength. The dashed lines illustrate the optimum chord in order to obtainthe rotor design target point for a blade without flow altering devices,and the full drawn lines illustrate the relationship between chordlength and local blade radius for a base part having a linear chordlength distribution and provided with flow altering devices. As shown inFIG. 24 a, the chord length may be lower than the chord length along theentire longitudinal extend of the longitudinal blade section. FIG. 24 billustrates a second embodiment, in which the chord length of the basepart is equal to the optimum chord length for a single cross-sectiononly, and where the remainder of the longitudinal blade section has achord length, which is lower than the optimum chord length.

FIG. 24 c illustrates an advantageous embodiment having a first part, inwhich the chord length is lower than the optimum chord length, a secondpart, in which the chord length is higher than the optimum chord length,and a third part, in which the chord length is higher than the optimumchord length. The linear chord length distribution may for instance bechosen as a median line to the optimum chord length distribution.

Embodiments having longitudinal blade sections with linearly dependentchord length are shown in FIGS. 9-11. In all these embodiments, thechord length of the blade is decreasing in the longitudinal or radialdirection of the blade towards the blade tip. However, the blade mayalso comprise a longitudinal blade part, in which the chord length isconstant. An embodiment of such a blade is shown in FIG. 25. The bladeis divided into a root region 430, a transition region 432, and anairfoil region 434. The airfoil region comprises a blade tip part 436, afirst longitudinal section 440, and a second longitudinal section 442.The first longitudinal section 440 of the blade is divided into a firstbase part 440 and a number of first flow altering devices 446, 448. Thesecond longitudinal section 442 of the blade is divided into a secondbase part 443 and a number of second flow altering devices 449. Thefirst base part 441 has a constant chord length along the entirelongitudinal extend of the first longitudinal section 440, whereas thesecond base part 443 has a chord length, which is linear decreasing inthe longitudinal direction of the second longitudinal section of theblade.

3.3 Base Part with Linearly Dependent Thickness

It is not shown in the figures, but it is advantageous that thethickness of the blade is also linearly dependent on the local radius ofthe blade. The particular longitudinal blade section may for instancehave the same relative profile along the entire longitudinal extent ofthe longitudinal blade section.

3.4 Base Part with Linearly Dependent Pre-Bend

Yet again, it may also be advantageous—especially with respect tomodularity—to design the base part of the particular longitudinalsection with a linear pre-bend, Δy, as illustrated in FIG. 26.

3.5 Blade Profiles

Normally, wind turbine blades designed according to conventional methodscomprise blade sections having a profile 150 with double curvaturepressure sides as shown in FIG. 27. The invention makes it possible tosimplify the design to profiles 250 without double curvatured pressuresides as shown in FIG. 28 and to compensate for the off-design profileby use of flow altering devices as previously explained.

Yet again, as shown in FIG. 30, the profile may for at least a part ofthe longitudinal section be simplified even further to a symmetricalprofile 350 having a chord 360 and camber 362, which are coincident.Such a blade profile has a relationship between lift coefficient andinflow angle, which crosses the origin of the coordinate system as shownin FIG. 29. This means that the graph is shifted towards higher inflowangles as compared to conventional non-symmetric blade profiles with apositive camber. This also implies that a particular lift coefficientmay be obtained at a higher angle of attack than for the conventionalblade profile. This is advantageous for embodiments having a reducedtwist compared to an optimum twist as explained in subsection 3.1. Inother words, the reduced twist and the symmetric profile at leastpartially compensate for each other.

This compensation may be exploited even further by providing at least apart of the longitudinal section with a profile 450 as shown in FIG. 32having a “negative camber”, i.e. a blade where the camber 462 is locatedcloser to the pressure side 452 of the blade than the suction side 454of the blade (or equivalently, where the camber 462 is located below thechord 460). A blade section has a negative lift coefficient for anincident airflow at an angle of attack of 0 degrees, i.e. the graphillustrating the relationship between lift coefficient and angle ofattack is shifted even further towards larger inflow angles. This inturn means that a particular lift coefficient may be obtained at an evenhigher angle of attack. The use of profiles having a negative camber maybe advantageous especially for blades having a very low twist or notwist, since such blades have blade sections, in which the operationalangle of attack at the rotor design point is very high. This isparticularly relevant for the inboard part of the airfoil region and thetransition region.

However, the camber 462 need not locally be located nearer the pressureside of the blade than the suction side of the blade along the entirechord of the profile as shown in FIG. 32. As shown in FIG. 33, it isalso possible to provide a blade section with profile 550 having anegative camber 562, in which a part of the camber is closer to thesuction side 554 of the blade than the pressure side of the blade 552(or above the chord 560) as long as the camber 562 on average is closerto the pressure side 552 than the suction side of the blade 554.

4 MODULARITY AND REUSE OF BLADE SECTIONS

As previously mentioned, the use of a simplified base part of thelongitudinal section of the blade makes it possible to use that basepart for several different types of blades and use flow altering devicesto compensate for the off-design characteristics of the base part.

FIG. 34 illustrates this principle. A wind turbine blade 410 is dividedinto a root region 430, a transition region 432, and an airfoil region434. The airfoil region 434 comprises a blade tip part 436, a firstlongitudinal section 440, and a second longitudinal section 442. Thefirst longitudinal section 440 of the blade is divided into a first basepart 441 and a number of first flow altering devices 446. The secondlongitudinal section 442 of the blade is divided into a second base part443 and a number of second flow altering devices 448. The first basepart 441 and the second base part 443 have profiles, which have asimplified structure with respect to for instance modularity of bladeparts and/or manufacturing of the base parts 441, 443, and which at therotor design point in itself deviate significantly from the target axialinduction factor and/or the target loading. The base parts 441 are heredepicted as having linearly dependent chord lengths, but advantageously,the sections also have a linearly dependent thickness and linearlydependent twist or no twist. Therefore, the longitudinal sections areprovided with the flow altering devices, which are here depicted as afirst slat 446 and a first flap 448, however; the flow altering devicesare not restricted to such flow altering devices only. The firstlongitudinal section 440 and the second longitudinal section 442 bothextend along at least 20% of the longitudinal length of the airfoilregion 434. The first longitudinal section or base part is located in afirst radial distance r₁.

The first base part 441 is reused for a second blade 410′, which alsocomprises a root region 430′, a transition region 432′, and an airfoilregion 434′. The airfoil region 434′ comprises a blade tip part 436′, afirst longitudinal section 440′, a second longitudinal section 442′having a second base part 443′, and a third longitudinal section ortransition section 445′ located between the first longitudinal section440′ and the transition region 432′. The first longitudinal section andfirst base part 441 of the second blade 410′ are located at a secondradial distance r₂. Therefore, the inflow conditions for the first basepart 441 are different from the first blade 410 and the second blade410′. Further, the target chord lengths (for base parts without flowaltering devices) need to be different in order obtain the target axialinduction factor and the target normal load. Consequently, the secondblade 410′ needs different flow altering devices 446′, 448′ than thefirst blade 410 in order to compensate for the off-design conditions.

The first base part 441 may be designed so that it without flow alteringdevices is suboptimum for both the first blade 410 and the second blade410′ as illustrated in FIG. 34. However, in principle, the first basepart 441 may be optimised for one of the two blades, so that flowaltering devices only are needed for the other of the two blades.

In principle, a first blade may be reused entirely for a second blade,for instance by providing the first blade with a hub extender as shownin FIG. 35. In this situation, substantially the entire hatched part ofthe second blade be encumbered with off-design conditions and a majorityof this section will need the use of flow altering devices,advantageously both the airfoil region and the transition region of thesecond blade.

5 OPERATION OF A WIND TURBINE WITH TRANSFORMABLE BLADES

In this section, the operation or control of a wind turbine comprising arotor having transformable blades according to the invention isdescribed. The principle is illustrated in FIG. 36, in which (a) showsthe first embodiment of the wind turbine blade according to theinvention (also shown in FIG. 9).

FIG. 36 (b) shows the loading as function of the local blade radius, inwhich the full drawn line is the target loading, and the dashed line isthe actual loading of the base part of the blade without flow alteringdevices. The blade pitch and rotational speed is adjusted so that anoutboard part of the blade for values above a radial distance r₀ meetsthe target loading at the rotor design point. In this situation, theactual target loading of the first base part 141 without flow alteringdevices is sub-optimum as illustrated with the dashed line. Therefore,the first base part 141 and possible other parts of the blade areprovided in order to compensate for the off-design conditions andadjusting the loading of the blade section to meet the target loadingalong the entire longitudinal extend thereof as shown in FIG. 36 (c).

Similar graphs may be plotted for the axial induction factor, as theblade section also needs to meet the target axial induction factor.

6 EXAMPLES

The following section describes a study of the transformable bladesconcept via examples. As previously mentioned the transformable bladecomprises a base part and an adjustable part. The adjustable partcomprises aero-devices or flow altering means, which are fitted to thebase part in order to adjust and meet the aerodynamic design target ofthe blade sections. By adjusting only the flow altering means, thetuning of section aerodynamics allows partial de-coupling between thestructural and aerodynamic design. The base part can be designed to haveoptimal structural properties and not necessarily optimal aerodynamics.Afterward, the flow altering devices will be designed to fill theaerodynamic gaps from non-optimum to near-optimum target conditions.Flow altering devices, among others, include flaps, slats, vortexgenerators and spoilers as described in Sec. 2.2.

Some of the presented graphs are somewhat coarse due to the use oflimited number of sampling points in the simulation tools used forverifying the transformable blade concept.

6.1 Blade of 40.3 m with Du-91-W2-250 Airfoil and No Twist

The first example takes a point of departure from a blade having alength of 40.3 meters and having an airfoil region with an ideal axialinduction factor (i.e. a=0.33) for every cross-section. The chord lengthc is shown as a function of the radial distance r_(t) from the tip inFIG. 37. The chord length distribution is substantially identical to theexisting LM40.3p blade manufactured and sold by the present applicant.It is seen that the chord length distribution is non-linearly dependentin the radial direction of the blade.

The transformable blade of the first example has a chord lengthdistribution identical to the LM40.3p blade but the outer 26 meters ofthe airfoil regions have been replaced with a DU-91-W2-250 airfoilprofile. Furthermore, the transformable blade is not twisted in thisregion. Finally, the relative thickness of the DU-91-W2-250 airfoilprofile is constant in the region. In the present example the relativethickness is 25%, i.e. the ratio between the maximum cross sectionthickness and the chord length at a given cross section is 25%. Thus,the airfoil region has been highly simplified in only having a singleairfoil shape along approximately 75% of the airfoil region and havingno twist.

FIG. 38 shows graphs of the twist Θ depicted as a function of the radialdistance r_(t) from the tip. A first graph 510 shows the twist for theideal blades, and another graph 520 shows the twist for thetransformable blade. It is seen that the twist of the transformableblade has a severe deviation from the ideal twist of several degrees onaverage over the radial extent of the blade.

FIG. 39 shows graphs of the inflow angle distribution along the span ofvarious 40.3 m blades (as a function of the radial distance r_(t) fromthe tip). The first and the second graphs 550, 560 show the distributionof the inflow angle φ for the ideal blade at wind speeds of 8 m/s and 10m/s, respectively, whereas the third and the fourth graphs 570, 580 showthe corresponding distribution of the inflow angle φ for thetransformable blade, respectively.

FIG. 40 shows graphs of the lift coefficient distribution along the spanof various 40.3 m blades (as a function of the radial distance r_(t)from the tip). The first and the second graphs 600, 610 show thedistribution of the lift coefficient c_(l) for the ideal blade at windspeeds of 8 m/s and 10 m/s, respectively, whereas the third and thefourth graphs 620, 630 show the corresponding distribution of the liftcoefficient c_(l) for the transformable blade, respectively.

FIG. 41 shows graphs of the axial induction factor distribution alongthe span of various 40.3 m blades (as a function of the radial distancer_(t) from the tip). The first and the second graphs 650, 660 show thedistribution of the axial induction factor a for the ideal blade at windspeeds of 8 m/s and 10 m/s, respectively, whereas the third and thefourth graphs 670, 680 show the corresponding distribution of the axialinduction factor a for the transformable blade, respectively. It is seenthat the axial induction factor over the airfoil region differs at least10% from the target axial induction factor of approximately 0.33.

By changing the outer part of the airfoil region with the DU-91-W2-250airfoil profile, the power production of a wind turbine using suchblades at wind speeds of 8 m/s and 10 m/s without the use of flowaltering devices is reduced by 3% compared to a wind turbine using theideal 40.3 meter blades. Furthermore, the change of profile leads to adeviation from an optimum power coefficient at a wind speed of 8 m/s ina region ranging from 10 meters to 26 meters from the tip (not shown inthe graphs). FIGS. 40 and 41 show that this deviation is caused by anoverloading in this region of the blade. Hence, this part of thetransformable blade should be provided with flow altering devicescapable of lowering the lift, thereby enhancing the mechanical poweroutput of a wind turbine using such blades.

FIGS. 40 and 41 also show that the outer 15 meters of the base part ofthe transformable blade are under-loaded at a wind speed of 10 m/s,causing a drop in mechanical power. Aero-devices, such as Gurney flapsand slats, adapted for increasing the lift would increase the mechanicalpower output. Unless actively controlled aero-devices are used, it isnot possible to achieve optimum conditions for both wind speeds, and acompromise has to be reached.

Overall, a highly simplified aerodynamic design of the base part of atransformable blade compared to the existing LM40.3p blade is achieved.The outer 26 meters of the base part of the transformable bladecomprises only a single relative airfoil profile and no twist. This doesnot only simplify the aerodynamic design of the base part, but alsosimplifies the manufacturing process of the blade and the manufacturingprocess of moulds for manufacturing blade parts of the blade. However,the base part in itself entails a power yield loss of approximately 3%compared to the ideal conditions. Flow altering devices are then used toadjust the aerodynamic characteristics to the target values and inparticular the axial induction factor, thus compensating fully for the3% loss.

FIG. 42 shows a first graph 710 showing the relative thickness of thetransformable blade compared to a second graph 700 showing the relativethickness of the existing LM40.3p blade as a function of the radialdistance r_(t) from the tip. It is seen that the relative thickness ofthe transformable blade is larger than the relative thickness of theLM40.3p blade. The bending stiffness of a wind turbine blade comprisinga shell body is proportional to the cube of the distance between theneutral axis of the blade and the shell body. This means that the shellbody of a relative thick profile may be thinner than a relative thinnerprofile and still obtain the same strength and stiffness. The shell bodyis typically made as a laminate structure comprising a matrix materialreinforced with fibres, such as glass fibres and/or carbon fibres. Inthe present example the redesigned part of the transformable blade is14.8% lighter than the corresponding part of the LM40.3p blade, and theoverall weight reduction is 7.7%. Thus, it is seen that also thematerial cost of a transformable blade may be reduced compared toexisting blades.

A similar study was made of a transformable blade having a relativethickness of 30% for the outer 32 meters. The weight of the redesignedpart is reduced with 21.4% compared to the corresponding part of anLM40.3p blade, and the overall weight reduction was 12.3%. Nonetheless,the mechanical power yield of a wind turbine using the transformableblade can be maximized from the use of the flow altering devices.

6.2 Group of Blades Having Identical Outboard Base Parts

The following examples demonstrate the use of an identical outboard basepart 830 for three different transformable blades 800, 810, 820 havingdifferent blade lengths as shown in FIG. 43. Further, the examplesdemonstrate one method of operating a wind turbine having a rotor withsuch transformable blades. The blades studied have lengths of 44.1,52.1, and 60.1 meters, respectively. The chord distributions 801, 811,821 of the three blades are shown in FIG. 44 as a function of the radialdistance r_(t) from the tip. In the following example, the transformableblades are studied for use on rotors with an identical tip speed of 70m/s for all three transformable blades 800, 810, 820. FIG. 45 shows thecorresponding graphs 802, 812, 822 of the operational inflow angle as afunction of the radial distance r_(t) from the tip, and FIG. 46 showsthe corresponding graphs 803, 813, 823 of the operational liftcoefficients as a function of the radial distance r_(t) from the tip.

If the different transformable blades 800, 810, 820 are to be operatedat the same tip speed, it is seen that the outboard 30 meters of thetransformable blades, where all blades share the same radial section,have different operational lift coefficients, and thus, it is clear thatthe product of the chord length and the lift coefficient is alsodifferent for the three blades. To achieve the operational condition forthe three blades, it is clear that flow altering devices are neededalong the entire radial extent of the outboard part for at least two ofthese blades to meet the target axial induction factor, whereas thethird blade may reach the target axial induction factor by design.

However, the lift coefficient distribution as shown in FIG. 46 may bealtered by changing the rotor parameters. Accordingly, by pitching theblades, it is possible to substantially translate the lift coefficienttowards higher or lower overall values, whereas it is possible to “tilt”the lift coefficient curves by altering the operational tip speed. Inorder to reduce the difference in operational lift coefficient of thethree transformable blades 800, 810, 820 in the outboard part of theblades, the three blades are in the following evaluated for tip speedsof 70 m/s, 75 m/s, and 80 m/s for the 44.1, 52.1, and 60.1 mtransformable blades, respectively.

FIG. 47 shows the corresponding graphs 804, 814, 824 of the inflow angleas a function of the radial distance r_(t) from the tip, and FIG. 48shows the corresponding graphs 805, 815, 825 of the lift coefficients asa function of the radial distance r_(t) from the tip. The resultsindicate a significant improvement in the operation lift coefficients.In the outboard 20 meters of the transformable blades, the liftcoefficients are substantially identical. Thus, no flow altering devicesare needed for this part to meet the target axial induction factor. Theinboard parts of the airfoil region of the base parts of thetransformable blades, however, need to be provided with flow alteringdevices in order to meet the target axial induction factor.

Accordingly, it is demonstrated that it is possible to adjust the pitchof the blade and the rotational speed of the rotor to meet the targetaxial induction factor of the outboard section, whereas the inboard partis provided with flow altering devices in order to meet the target axialinduction factor of the inboard section of the airfoil region.

6.3 Group of Staggered Blades

As with the previous example, a first transformable blade 900, a secondtransformable blade 910, and a third transformable blade 920 having ablade length of 44.1 m, 52.1 m, and 60.1 m, respectively, are studied.The three transformable blades 900, 910, 920 share an identical midboardor inboard 930 base part. Thus, the three transformable blades arestaggered as shown in FIG. 49. It is seen that the transformable blades900, 910, 920 have different tip parts and root (and transition) parts.

FIG. 50 depicts graphs 901, 911, 921 of the chord length distributionfor the three transformable blades 900, 910, 920, respectively, as afunction of the radial distance r_(t) from the tip. It is seen that theblades have a common section with a linear chord length variation andstaggered tip locations. The section 930 of the 60.1 m transformableblade 920 is staggered 2.5 and 5 m from the identical sections of the52.1 m transformable blade 910 and the 44.1 m transformable blade 920,respectively. This means that the shared section 930 for instance may belocated 15 m, 12.5 m, and 10 m from the tip of the three blades,respectively.

In the following graphs, the tip of the 44.1, 52.1 and 60.1 m blades isplaced at 0, 2.5 and 5 m, respectively. This simplifies the comparisonof results between identical sections found at the same horizontal-axislocation. The following computations are carried out imposing a tipspeed of 75 m/s on all blades.

FIG. 51 shows the corresponding graphs 902, 912, 922 of the inflow angleas a function of the radial distance r_(t) from the tip, and FIG. 52shows the corresponding graphs 903, 913, 923 of the lift coefficients asa function of the radial distance r_(t) from the tip. It is seen thatthe lift coefficients of sections ranging from 10 to 20 meters from theblade tip of the three transformable blades 900, 910, 920 show excellentagreement, which means that flow altering devices are not needed in thisregion. Further, it is seen that the 52.1 and 60.1 m transformableblades 910, 920 show agreement of the aerodynamic features in the rangefrom 20 to 30 meter from the tip. Thus, if the shared blade section 930is optimised for the 52.1 and 60.1 m transformable blades 910, 920, onlythe 44.1 m transformable blade 900 need to be fitted with flow alteringdevices in this region in order to meet the target axial inductionfactor. However, it is seen that in the section beyond 30 meters fromthe blade tip, the aerodynamic operational parameters of the threetransformable blades are significantly different, which means that flowaltering devices are needed in the inboard part of at least two of theseblades in order to meet the target axial induction factor.

Thus, it is demonstrated that a section of a blade can be shared for astaggered group of blades at a midboard or midspan part of the blades insuch a way that a large part of the shared section does not need to beprovided with flow altering devices to meet the target axial inductionfactor. Accordingly, the pitch of the blades and the rotational speed ofthe rotor are adjusted to meet the target axial induction factor of themidboard section, whereas the inboard part (and possibly the outboardpart) is provided with flow altering devices to meet the target axialinduction factor of the inboard section (and the outboard part) of theairfoil region. Furthermore, by staggering the blades, it is alsopossible to obtain nearly identical flapwise bending moments (notshown). This means that not only the outer contour of the shared sectionis the same but that the laminate structure may also be provided withthe same design and thickness.

FINAL REMARKS

The invention has been described with reference to a preferredembodiment. However, the scope of the invention is not limited to theillustrated embodiment, and alterations and modifications can be carriedout without deviating from the scope of the invention.

LIST OF REFERENCE NUMERALS

-   2 wind turbine-   4 tower-   6 nacelle-   8 hub-   10, 410 blade-   14 blade tip-   16 blade root-   18 leading edge-   20 trailing edge-   30, 130, 230, 330, 430 root region-   32, 132, 232, 332, 432 transition region-   34, 134, 234, 334, 434 airfoil region-   36, 136, 236, 336, 436 tip region-   38 radial blade sections-   140, 240, 340 first longitudinal section-   141, 241, 341, 441 first base part-   242, 342 second longitudinal section-   243, 343, 443 second base part-   344 transition section-   345 transition base part-   146-149, 246, 248, 346-349, 446, 448 flow altering devices-   50, 150, 250, 350, 450, 550 airfoil profile-   52, 452, 552 pressure side-   54, 454, 554 suction side-   56 leading edge-   58 trailing edge-   60, 360, 460, 560 chord-   62, 362, 462, 562 camber line/median line-   64 direction of rotation-   66 lift-   68 drag-   70 resultant aerodynamic force-   72 axial force (thrust)-   74 tangential force-   75 moment coefficient-   76 optimum angle of attack for airfoil profile of radial blade    section without flow altering devices-   78 average angle of attack for radial blade section-   80, 180-182, 280, 380, 480 flow altering devices-   a axial induction factor-   a′ tangential induction factor-   b vane angle-   c chord length-   c_(d) drag coefficient-   c_(l) lift coefficient-   c_(m) moment coefficient-   d_(t) position of maximum thickness-   d_(f) position of maximum camber-   f camber-   h vane height-   l vane length-   r·ω rotational velocity-   r local radius, radial distance from blade root-   r_(t) radial distance from blade tip-   s vane spacing-   t thickness-   z vane pair spacing-   x tip speed ratio-   B number of blades-   N normal force-   P power-   R rotor radius-   T tangential force-   X tip speed-   V design point wind speed-   v_(a) axial velocity-   v_(w) wind speed-   v_(r), W, v_(inflow) resultant speed, inflow speed-   α angle of attack-   ω, Ω rotational speed of rotor-   Θ, θ twist, pitch-   Δy prebend

The invention claimed is:
 1. A method of manufacturing a wind turbineblade by using the design of a longitudinal segment of a first windturbine blade for the design of a second wind turbine blade, wherein thefirst wind turbine blade and the second wind turbine blade each comprisea longitudinally extending base part having: a profiled contourcomprising a pressure side and a suction side as well as a leading edgeand a trailing edge with a chord extending between the leading edge andthe trailing edge, the profiled contour generating a lift when beingimpacted by an incident airflow, the profiled contour in the radialdirection being divided into a root region with a substantially circularor elliptical profile closest to a hub, an airfoil region with a liftgenerating profile furthest away from the hub, and a transition regionbetween the root region and the airfoil region, the transition regionhaving a profile gradually changing in the radial direction from thecircular or elliptical profile of the root region to the lift generatingprofile of the airfoil region, characterised in that the methodcomprises the steps of: a) taking a first blade design of a first basepart of a first longitudinal section of the airfoil region of a firstblade, b) using the first blade design for the first base part on afirst longitudinal section of the airfoil region of a second blade, sothat an axial induction factor of the first base part on the secondblade without flow altering devices at a rotor design point deviatesfrom a target induction factor, and c) providing the first longitudinalsection of the second blade with first flow altering devices so as toadjust the aerodynamic properties of the first longitudinal segment andobtain correct lift and drag coefficients at a given angle of attach tosubstantially meet the target axial induction factor at the design pointon the second blade, wherein the first longitudinal segment extendsalong at least 20% of a longitudinal extent of the airfoil region of thesecond blade.
 2. A method according to claim 1, wherein the firstlongitudinal segment of the first blade is located in a first radialdistance from a root end of the first blade, and wherein the firstlongitudinal segment of the second blade is located in a second radialdistance from a root end of the second blade, and wherein the firstradial distance is different from the second radial distance.
 3. Amethod according to claim 2, wherein the first base part is optimisedfor operation at the first radial distance.
 4. A method according toclaim 2, wherein the first base part has a profile shape which issubstantially a hybrid between a first profile shape for obtaining atarget axial induction factor at a first radial distance at the rotordesign point and a second profile shape for obtaining a target inductionat a second radial distance at the rotor design point.
 5. A methodaccording to claim 4, wherein the first base part has a profile shapewhich is substantially an average between the first profile shape andthe second profile shape.
 6. A method according to claim 1, wherein stepb) is carried out by connecting a hub extender to the first blade.
 7. Amethod according to claim 1, wherein the flow altering devices comprisedevices chosen from the group of: multi element sections, wherein themulti element sections are selected from the group consisting of a slat,a flap, surface mounted elements, and combinations thereof, wherein thesurface mounted elements are a leading edge element or a surface mountedflap, which alter an overall envelope of the first longitudinal segmentof the blade.
 8. A method according to claim 7, wherein the flowaltering devices additionally comprise boundary layer control means,wherein the boundary layer control means are selected from the groupconsisting of holes for ventilation, a slot for ventilation, vortexgenerators, a Gurney flap and combinations thereof.
 9. A group of windturbine blades comprising at least a first wind turbine blade and asecond wind turbine blade, wherein the first wind turbine blade and thesecond wind turbine blade each comprise a longitudinally extending basepart having: a profiled contour comprising a pressure side and a suctionside as well as a leading edge and a trailing edge with a chordextending between the leading edge and the trailing edge, the profiledcontour generating a lift when being impacted by an incident airflow,the profiled contour in the radial direction being divided into a rootregion with a substantially circular or elliptical profile closest to ahub, an airfoil region with a lift generating profile furthest away fromthe hub, and a transition region between the root region and the airfoilregion, the transition region having a profile gradually changing in theradial direction from the circular or elliptical profile of the rootregion to the lift generating profile of the airfoil region,characterised in that the first wind turbine blade and the second windturbine blade comprise a first longitudinal segment having an asubstantial identical first base part, wherein the first longitudinalsegment extends along at least 20% of a longitudinal extent of theairfoil region of the second wind turbine blade, and wherein aninduction factor of the first base part of the second blade without flowaltering devices at a rotor design point deviates from a target axialinduction factor, and wherein the first longitudinal segment of thesecond blade is provided with a number of first flow altering devicesarranged so as to adjust the lift and drag coefficients of the firstlongitudinal segment to substantially meet the target axial inductionfactor at the rotor design point; wherein the blades are manufacturedaccording to the method of claim
 1. 10. A group of wind turbine bladesaccording to claim 9, wherein the second wind turbine blade comprisesthe base part of the first wind turbine blade and is further providedwith a hub extender.
 11. A group of wind turbines comprising at least afirst wind turbine and a second wind turbine, wherein the first windturbine comprises a rotor with two or three first wind turbine bladesaccording to claim 9, and the second wind turbine comprises a rotor withtwo or three first wind turbine blades according to claim 9.